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 0a934642569bc41e2e79d61bd427a3bf161fee5f..1aa4d45a702840eaf54555fcc725293cb050bf12 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -435,6 +435,7 @@ private:
     } fHWGeometryState;
 
     struct {
+        GrBlendEquation fEquation;
         GrBlendCoeff    fSrcCoeff;
         GrBlendCoeff    fDstCoeff;
         GrColor         fConstColor;
@@ -442,6 +443,7 @@ private:
         TriState        fEnabled;
 
         void invalidate() {
+            fEquation = kInvalid_GrBlendEquation;
             fSrcCoeff = kInvalid_GrBlendCoeff;
             fDstCoeff = kInvalid_GrBlendCoeff;
             fConstColorValid = false;