Key shader on whether frag pos read is relative to top-left or bottom-left

src/gpu/gl/GrGLProgramDesc.h

Index: src/gpu/gl/GrGLProgramDesc.h
diff --git a/src/gpu/gl/GrGLProgramDesc.h b/src/gpu/gl/GrGLProgramDesc.h
index 45714a0dc67bf4e45e35b9b58a411835626e7883..b49cb722c017f0f5b67bcbcb5e3c69bf9ce755db 100644
--- a/src/gpu/gl/GrGLProgramDesc.h
+++ b/src/gpu/gl/GrGLProgramDesc.h
@@ -79,7 +79,7 @@ private:
         // (1 - colorRGB) as the secondary output. Only set if dual source blending is supported.
         kSecondaryCoverageISC_CoverageOutput,
         // Combines the coverage, dst, and color as coverage * color + (1 - coverage) * dst. This
-        // can only be set if fDstRead is set.
+        // can only be set if fDstReadKey is non-zero.
         kCombineWithDst_CoverageOutput,
 
         kCoverageOutputCnt
@@ -105,9 +105,12 @@ private:
     bool                     fExperimentalGS;
 #endif
 
-    GrGLShaderBuilder::DstReadKey fDstRead;             // set by GrGLShaderBuilder if there
+    GrGLShaderBuilder::DstReadKey fDstReadKey;          // set by GrGLShaderBuilder if there
                                                         // are effects that must read the dst.
                                                         // Otherwise, 0.
+    GrGLShaderBuilder::FragPosKey fFragPosKey;          // set by GrGLShaderBuilder if there are
+                                                        // effects that read the fragment position.
+                                                        // Otherwise, 0.
 
     // should the FS discard if the coverage is zero (to avoid stencil manipulation)
     SkBool8                     fDiscardIfZeroCoverage;