Translate .Load() to imageLoad() for Vulkan (#967)

* Translate .Load() to imageLoad() for Vulkan

We were already emitting calls to `imageLoad()` and `imageStore` when a `RWTexture*` was used with `operator[]`:

```hlsl
RWTexture2D<float> myTex;
...
float value = myTex[xy]; // becomes an imageLoad
myTex[xy] = value;       // becomes an imageStore
```

However, we were *not* correctly handling the translation of an explicit `.Load()` operation:

```hlsl
float value = myTex.Load(xy);
```

The `.Load()` operation was being translated to a GLSL `texelFetch` as it would be a for a `Texture2D`, and not to an `imageLoad()` as would make sense for a `RWTexture2D` (which becomes a GLSL `image2D`).

This fix is confined to the stdlib, and is mostly a matter of emitting either `texelFetch` or `imageLoad` as the GLSL function name depending on the "access" of the resource type. It is messy code, but straightforward.

One extra detail was that there had been logic to emit a `, 0` argument in the `texelFetch` calls in the non-read-only case, because `texelFetch` usualy requires an explicit mip-level argument and `.Load()` on a `RWTexture*` doesn't recieve an LOD parameter. This is a non-issue now that we are calling `imageLoad()` instead, because `imageLoad` doesn't need/want the extra argument.

* fixup: change test baseline based on recent GLSL output changes

* fixup: review feedback

1 files changed, 7 insertions, 5 deletions

diff --git a/source/slang/core.meta.slang b/source/slang/core.meta.slang
index a3d73f66a..bc9c9b50a 100644
--- a/source/slang/core.meta.slang
+++ b/source/slang/core.meta.slang
@@ -687,6 +687,8 @@ for (int tt = 0; tt < kBaseTextureTypeCount; ++tt)
 
                 int loadCoordCount = kBaseTextureTypes[tt].coordCount + isArray + (needsMipLevel?1:0);
 
+                char const* glslFuncName = (access == SLANG_RESOURCE_ACCESS_READ) ? "texelFetch" : "imageLoad";
+
                 // When translating to GLSL, we need to break apart the `location` argument.
                 //
                 // TODO: this should realy be handled by having this member actually get lowered!
@@ -703,19 +705,19 @@ for (int tt = 0; tt < kBaseTextureTypeCount; ++tt)
                 if (isMultisample)
                 {
                     sb << "__glsl_extension(GL_EXT_samplerless_texture_functions)";
-                    sb << "__target_intrinsic(glsl, \"$ctexelFetch($0, $1, $3)$z\")\n";
+                    sb << "__target_intrinsic(glsl, \"$c" << glslFuncName << "($0, $1, $3)$z\")\n";
                 }
                 else
                 {
                     sb << "__glsl_extension(GL_EXT_samplerless_texture_functions)";
-                    sb << "__target_intrinsic(glsl, \"$ctexelFetch($0, ";
+                    sb << "__target_intrinsic(glsl, \"$c" << glslFuncName << "($0, ";
                     if( needsMipLevel )
                     {
                         sb << "($1)." << kGLSLLoadCoordsSwizzle[loadCoordCount] << ", ($1)." << kGLSLLoadLODSwizzle[loadCoordCount];
                     }
                     else
                     {
-                        sb << "$1, 0";
+                        sb << "$1";
                     }
                     sb << ")$z\")\n";
                 }
@@ -730,12 +732,12 @@ for (int tt = 0; tt < kBaseTextureTypeCount; ++tt)
                 if (isMultisample)
                 {
                     sb << "__glsl_extension(GL_EXT_samplerless_texture_functions)";
-                    sb << "__target_intrinsic(glsl, \"$ctexelFetchOffset($0, $0, $1, $2)$z\")\n";
+                    sb << "__target_intrinsic(glsl, \"$c" << glslFuncName << "($0, $0, $1, $2)$z\")\n";
                 }
                 else
                 {
                     sb << "__glsl_extension(GL_EXT_samplerless_texture_functions)";
-                    sb << "__target_intrinsic(glsl, \"$ctexelFetch($0, ";
+                    sb << "__target_intrinsic(glsl, \"$c" << glslFuncName << "($0, ";
                     if( needsMipLevel )
                     {
                         sb << "($1)." << kGLSLLoadCoordsSwizzle[loadCoordCount] << ", ($1)." << kGLSLLoadLODSwizzle[loadCoordCount];