Add "sample locations" feature to GrProcessor

src/gpu/GrProgramDesc.h

Index: src/gpu/GrProgramDesc.h
diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index 63e060ee6288f22de3cf1f5cea870a9f06012cfb..53c461ccad6713b7d038acedacf913a2ac197215 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -79,6 +79,9 @@ public:
         int8_t                      fColorEffectCnt;
         int8_t                      fCoverageEffectCnt;
         uint8_t                     fIgnoresCoverage;
+        // Set to uniquely identify the pipeline's sample pattern, or 0 if the shader does not use
+        // sample locations. (We can probably get away with 8 bits here if we ever need more space.)
+        uint16_t                    fSamplePatternKey;
     };
 
     int numColorEffects() const {