Handle `flat` interpolation cases in cross compilation

Fixes #104

- Map HLSL `nointerpolation` to GLSL `flat`

- When lowering a `struct` type varying input/output, look for interpolation modifiers along the "chain" from the leaf field up to the original shader input variable (and take the first one found)
  - Not sure if this is strictly needed, but it seems like a reasonable policy

- Add `flat` to varying input of integer type, with no other interpolation modifier
  - Note: I do *not* do anything to ignore a manually imposed interpolation modifier that might be incorrect

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/cross-compile/integer-input.slang b/tests/cross-compile/integer-input.slang
new file mode 100644
index 000000000..2069091c6
--- /dev/null
+++ b/tests/cross-compile/integer-input.slang
@@ -0,0 +1,11 @@
+//TEST:CROSS_COMPILE: -profile ps_5_0 -entry main -target spirv-assembly
+
+struct VS_OUT
+{
+	uint drawID : DRAW_ID;	
+};
+
+float4 main(VS_OUT vsOut) : SV_Target
+{
+	return float4(float(vsOut.drawID));
+}