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..ec6447d622698a328d90cb1bad8915813888607f 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -70,9 +70,11 @@ public:
     }
 
     struct KeyHeader {
-        // Set by GrGLShaderBuilder if there are effects that read the fragment position. Otherwise,
-        // 0.
-        uint8_t                     fFragPosKey;
+        // Set to uniquely identify the rt's origin, or 0 if the shader does not require this info.
+        uint8_t                     fSurfaceOriginKey;
+        // Set to uniquely identify the sample pattern, or 0 if the shader doesn't use sample
+        // locations.
+        uint8_t                     fSamplePatternKey;
         // Set to uniquely idenitify any swizzling of the shader's output color(s).
         uint8_t                     fOutputSwizzle;
         uint8_t                     fSnapVerticesToPixelCenters;