Implement support for KHR_blend_equation_advanced

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index b2512977037e143e33d5c7c77cac1581754cd422..0accc0ca791961ad22e4e181ef87d39b6c6143e8 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -437,6 +437,7 @@ private:
     } fHWGeometryState;
 
     struct {
+        GrBlendEquation fEquation;
         GrBlendCoeff    fSrcCoeff;
         GrBlendCoeff    fDstCoeff;
         GrColor         fConstColor;
@@ -444,6 +445,7 @@ private:
         TriState        fEnabled;
 
         void invalidate() {
+            fEquation = kInvalid_GrBlendEquation;
             fSrcCoeff = kInvalid_GrBlendCoeff;
             fDstCoeff = kInvalid_GrBlendCoeff;
             fConstColorValid = false;