Track stage for varying sub-fields (#842)

Fixes #841

This reverts a small change made in #815 that seemed innocent at the time: we stopped tracking an explicit `Stage` to go with every `VarLayout` that is part of an entry-point varying parameter, and instead only associated the stage with the top-level parameter. That change ended up breaking the logic to emit the `flat` modifier automatically for integer type fragment-shader inputs for GLSL, but we didn't have a regression test to catch that case.

This change adds a regression test to cover this case, and adds the small number of lines that were removed from `parameter-binding.cpp`.
A few other test outputs had to be updated for the change (these are outputs that were changed in #815 for the same reason).

1 files changed, 13 insertions, 0 deletions

diff --git a/source/slang/parameter-binding.cpp b/source/slang/parameter-binding.cpp
index 89226cdd9..f0abfe31c 100644
--- a/source/slang/parameter-binding.cpp
+++ b/source/slang/parameter-binding.cpp
@@ -2016,6 +2016,19 @@ static RefPtr<TypeLayout> processEntryPointVaryingParameter(
     EntryPointParameterState const& state,
     RefPtr<VarLayout>               varLayout)
 {
+    // Make sure to associate a stage with every
+    // varying parameter (including sub-fields of
+    // `struct`-type parameters), since downstream
+    // code generation will need to look at the
+    // stage (possibly on individual leaf fields) to
+    // decide when to emit things like the `flat`
+    // interpolation modifier.
+    //
+    if( varLayout )
+    {
+        varLayout->stage = state.stage;
+    }
+
     // The default handling of varying parameters should not apply
     // to geometry shader output streams; they have their own special rules.
     if( auto gsStreamType = as<HLSLStreamOutputType>(type) )