Dawn: work around a fragcoord.w issue on D3D12.

D3D12 supplies w in fragcoord.w, while other backends supply 1/w.
This may be better handled in Dawn or SPIRV-Cross, but for now we'll work around the issue here.

Bug: skia:10475
Change-Id: I8f32f71f8f6b454365fbd0728fe063de2c150b8a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300711
Commit-Queue: Stephen White <[email protected]>
Reviewed-by: Greg Daniel <[email protected]>

Author: SenorBlanco

src/gpu/dawn/GrDawnProgramBuilder.cpp

@@ -23,6 +23,12 @@ static SkSL::String sksl_to_spirv(const GrDawnGpu* gpu, const char* shaderString
     settings.fRTHeightOffset = rtHeightOffset;
     settings.fRTHeightBinding = 0;
     settings.fRTHeightSet = 0;
+#ifdef SK_BUILD_FOR_WIN
+    // Work around the fact that D3D12 gives w in fragcoord.w, while the other APIs give 1/w.
+    // This difference may be better handled by Dawn, at which point this workaround can be removed.
+    // (See http://skbug.com/10475).
+    settings.fInverseW = true;
+#endif
     std::unique_ptr<SkSL::Program> program = gpu->shaderCompiler()->convertProgram(
         kind,
         shaderString,