Add API to query stage of varying parameter (#302)

Fixes #301

The problem here is that if you have input GLSL code like:

```glsl
// example.vs
in vec3 pos;
```

and:

```glsl
// example.fs
in vec3 worldPos;
```

Then both `pos` and `worldPos` are reflected as global variables (parameters of the *program*), which both get bound to "varying input" resources, but there is no way to tell through the API that `pos` is a vertex parameter while `worldPos` is a fragment one.

The original request in issue #301 was to expose parameters like this not as a global variables, but rather as parameters of the entry point in their specific file. That is, treat it as if the user had written, e.g.:

```glsl
// example.vs
void vsMain(in vec3 pos) { ... }
```

Doing that would unify the GLSL and HLSL/Slang cases a bit, but would require the Slang reflection API to lie about the structure of code the user wrote. At a more basic level, that would have been hard to implement because the current reflection API just exposes the underlying AST, and the AST *needs* to leave `pos` at the global scope so that when we go and spit GLSL back out we retain the original structure.

This PR implements a more simplistic solution, where the user is allowed to query the stage that a varying parameter "belongs" to. For right now I'm only enabling this to work for varying parameters (but it doesn't care if they are entry-point or global-scope varyings). Despite what I said on #301, this should work for both the top-level parameter's variable layout, *and* any variable layouts for fields within its type reflection.

In terms of implementation, I took the simple but wasteful route: every `VarLayout` now has a `stage` field that is by default initialized to `SLANG_STAGE_NONE`. When collecting varying parameters, I take advantage of the fact that everything bottlenecks through `processEntryPointParameter()` which takes an `EntryPointParameterState` so that I can set the `VarLayout::stage` field for any varying parameter in one place.

While I was making this change, I also did a bit of cleanup so that the "official" names for the varying parameter categories are `VARYING_INPUT` and `VARYING_OUTPUT`, with `VERTEX_INPUT` and `FRAGMENT_OUTPUT` being "deprecated" in principle. I didn't do the bulk rename inside the codebase yet.

2 files changed, 26 insertions, 13 deletions

diff --git a/tests/reflection/sample-rate-input.glsl.expected b/tests/reflection/sample-rate-input.glsl.expected
index 5800a3630..53e0b6e17 100644
--- a/tests/reflection/sample-rate-input.glsl.expected
+++ b/tests/reflection/sample-rate-input.glsl.expected
@@ -21,7 +21,8 @@ standard output = {
         },
         {
             "name": "uv",
-            "binding": {"kind": "vertexInput", "index": 0},
+            "stage": "fragment",
+            "binding": {"kind": "varyingInput", "index": 0},
             "type": {
                 "kind": "vector",
                 "elementCount": 2,
@@ -33,7 +34,8 @@ standard output = {
         },
         {
             "name": "c",
-            "binding": {"kind": "fragmentOutput", "index": 0},
+            "stage": "fragment",
+            "binding": {"kind": "varyingOutput", "index": 0},
             "type": {
                 "kind": "vector",
                 "elementCount": 4,
diff --git a/tests/reflection/vertex-input-semantics.hlsl.expected b/tests/reflection/vertex-input-semantics.hlsl.expected
index a3747a86a..06b7bc95a 100644
--- a/tests/reflection/vertex-input-semantics.hlsl.expected
+++ b/tests/reflection/vertex-input-semantics.hlsl.expected
@@ -13,7 +13,8 @@ standard output = {
             "parameters": [
                 {
                     "name": "a",
-                    "binding": {"kind": "vertexInput", "index": 0},
+                    "stage": "vertex",
+                    "binding": {"kind": "varyingInput", "index": 0},
                     "semanticName": "A",
                     "type": {
                         "kind": "vector",
@@ -26,7 +27,8 @@ standard output = {
                 },
                 {
                     "name": "b",
-                    "binding": {"kind": "vertexInput", "index": 1, "count": 3},
+                    "stage": "vertex",
+                    "binding": {"kind": "varyingInput", "index": 1, "count": 3},
                     "semanticName": "B",
                     "type": {
                         "kind": "struct",
@@ -42,7 +44,8 @@ standard output = {
                                         "scalarType": "int32"
                                     }
                                 },
-                                "binding": {"kind": "vertexInput", "index": 0},
+                                "stage": "vertex",
+                                "binding": {"kind": "varyingInput", "index": 0},
                                 "semanticName": "B"
                             },
                             {
@@ -61,7 +64,8 @@ standard output = {
                                                     "scalarType": "float32"
                                                 }
                                             },
-                                            "binding": {"kind": "vertexInput", "index": 0},
+                                            "stage": "vertex",
+                                            "binding": {"kind": "varyingInput", "index": 0},
                                             "semanticName": "B",
                                             "semanticIndex": 1
                                         },
@@ -75,13 +79,15 @@ standard output = {
                                                     "scalarType": "float32"
                                                 }
                                             },
-                                            "binding": {"kind": "vertexInput", "index": 1},
+                                            "stage": "vertex",
+                                            "binding": {"kind": "varyingInput", "index": 1},
                                             "semanticName": "B",
                                             "semanticIndex": 2
                                         }
                                     ]
                                 },
-                                "binding": {"kind": "vertexInput", "index": 1, "count": 2},
+                                "stage": "vertex",
+                                "binding": {"kind": "varyingInput", "index": 1, "count": 2},
                                 "semanticName": "B",
                                 "semanticIndex": 1
                             }
@@ -90,7 +96,8 @@ standard output = {
                 },
                 {
                     "name": "c",
-                    "binding": {"kind": "vertexInput", "index": 4, "count": 3},
+                    "stage": "vertex",
+                    "binding": {"kind": "varyingInput", "index": 4, "count": 3},
                     "type": {
                         "kind": "struct",
                         "name": "C",
@@ -111,7 +118,8 @@ standard output = {
                                                     "scalarType": "float32"
                                                 }
                                             },
-                                            "binding": {"kind": "vertexInput", "index": 0},
+                                            "stage": "vertex",
+                                            "binding": {"kind": "varyingInput", "index": 0},
                                             "semanticName": "CX"
                                         },
                                         {
@@ -124,13 +132,15 @@ standard output = {
                                                     "scalarType": "float32"
                                                 }
                                             },
-                                            "binding": {"kind": "vertexInput", "index": 1},
+                                            "stage": "vertex",
+                                            "binding": {"kind": "varyingInput", "index": 1},
                                             "semanticName": "CX",
                                             "semanticIndex": 1
                                         }
                                     ]
                                 },
-                                "binding": {"kind": "vertexInput", "index": 0, "count": 2},
+                                "stage": "vertex",
+                                "binding": {"kind": "varyingInput", "index": 0, "count": 2},
                                 "semanticName": "CX"
                             },
                             {
@@ -143,7 +153,8 @@ standard output = {
                                         "scalarType": "int32"
                                     }
                                 },
-                                "binding": {"kind": "vertexInput", "index": 2},
+                                "stage": "vertex",
+                                "binding": {"kind": "varyingInput", "index": 2},
                                 "semanticName": "CY"
                             }
                         ]