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

src/gpu/gl/GrGLShaderBuilder.h

Index: src/gpu/gl/GrGLShaderBuilder.h
diff --git a/src/gpu/gl/GrGLShaderBuilder.h b/src/gpu/gl/GrGLShaderBuilder.h
index 6e990aad01b17c56150221d47cda0b869a250545..b67846a621e05b5feb9e0cf26d2a1c64e78a42be 100644
--- a/src/gpu/gl/GrGLShaderBuilder.h
+++ b/src/gpu/gl/GrGLShaderBuilder.h
@@ -193,12 +193,18 @@ public:
                                                                  const GrGLCaps&);
 
     typedef uint8_t DstReadKey;
+    typedef uint8_t FragPosKey;
 
     /**  Returns a key for adding code to read the copy-of-dst color in service of effects that
          require reading the dst. It must not return 0 because 0 indicates that there is no dst
-         copy read at all. */
+         copy read at all (in which case this function should not be called). */
     static DstReadKey KeyForDstRead(const GrTexture* dstCopy, const GrGLCaps&);
 
+    /** Returns a key for reading the fragment location. This should only be called if there is an
+        effect that will requires the fragment position. If the fragment position is not required,
+        the key is 0. */
+    static FragPosKey KeyForFragmentPosition(const GrRenderTarget* dst, const GrGLCaps&);
+
     /** If texture swizzling is available using tex parameters then it is preferred over mangling
         the generated shader code. This potentially allows greater reuse of cached shaders. */
     static const GrGLenum* GetTexParamSwizzle(GrPixelConfig config, const GrGLCaps& caps);
@@ -424,6 +430,12 @@ private:
         kTopLeftOrigin_DstReadKeyBit  = 0x4, // Set if dst-copy origin is top-left.
     };
 
+    enum {
+        kNoFragPosRead_FragPosKey           = 0,  // The fragment positition will not be needed.
+        kTopLeftFragPosRead_FragPosKey      = 0x1,// Read frag pos relative to top-left.
+        kBottomLeftFragPosRead_FragPosKey   = 0x2,// Read frag pos relative to bottom-left.
+    };
+
     const GrGLContextInfo&              fCtxInfo;
     GrGLUniformManager&                 fUniformManager;
     uint32_t                            fFSFeaturesAddedMask;
@@ -443,6 +455,8 @@ private:
     GrGLUniformManager::UniformHandle   fDstCopyTopLeftUniform;
     GrGLUniformManager::UniformHandle   fDstCopyScaleUniform;
 
+    bool                                fTopLeftFragPosRead;
+
     SkSTArray<10, AttributePair, true>  fEffectAttributes;
 
     GrGLShaderVar*                      fPositionVar;