Support SampleId and SamplePosition builtins

The sampleRateShading feature adds new builtins variables which have
to be supported: BuiltInSampleId and BuiltInSamplePosition. This cl
sets the values for these builtins properly and turns on per sample
rendering where either of these is persent in the shader.

Bug: b/171415086
Change-Id: I959e80722493d12213be349019a0cc3a108a7596
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51735
Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/Pipeline/PixelProgram.cpp

diff --git a/src/Pipeline/PixelProgram.cpp b/src/Pipeline/PixelProgram.cpp
index 5066996..bdc3da7 100644
--- a/src/Pipeline/PixelProgram.cpp
+++ b/src/Pipeline/PixelProgram.cpp
@@ -170,6 +170,22 @@
 			routine.getVariable(it->second.Id)[it->second.FirstComponent + i] = Float4(0);
 	}
 
+	it = spirvShader->inputBuiltins.find(spv::BuiltInSampleId);
+	if(it != spirvShader->inputBuiltins.end())
+	{
+		routine.getVariable(it->second.Id)[it->second.FirstComponent] =
+		    As<SIMD::Float>(SIMD::Int((sampleId >= 0) ? sampleId : 0));
+	}
+
+	it = spirvShader->inputBuiltins.find(spv::BuiltInSamplePosition);
+	if(it != spirvShader->inputBuiltins.end())
+	{
+		routine.getVariable(it->second.Id)[it->second.FirstComponent + 0] =
+		    SIMD::Float(((sampleId >= 0) && (state.multiSampleCount > 1)) ? Constants::VkSampleLocations4[sampleId][0] : 0.5f);
+		routine.getVariable(it->second.Id)[it->second.FirstComponent + 1] =
+		    SIMD::Float(((sampleId >= 0) && (state.multiSampleCount > 1)) ? Constants::VkSampleLocations4[sampleId][1] : 0.5f);
+	}
+
 	// Note: all lanes initially active to facilitate derivatives etc. Actual coverage is
 	// handled separately, through the cMask.
 	auto activeLaneMask = SIMD::Int(0xFFFFFFFF);