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 5357142ec9d302358dd39b31f828bf4232e3cffd..56a49d5ab7d1bd167d3bb2209e16b74b20a21185 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;