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 4d74a758cea0c134a8d3bdf1d3364b8d684aaffb..6e82a8c15cec3047d66974d480d010694e082365 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -436,6 +436,7 @@ private:
     } fHWGeometryState;
 
     struct {
+        GrBlendEquation fEquation;
         GrBlendCoeff    fSrcCoeff;
         GrBlendCoeff    fDstCoeff;
         GrColor         fConstColor;
@@ -443,6 +444,7 @@ private:
         TriState        fEnabled;
 
         void invalidate() {
+            fEquation = kInvalid_GrBlendEquation;
             fSrcCoeff = kInvalid_GrBlendCoeff;
             fDstCoeff = kInvalid_GrBlendCoeff;
             fConstColorValid = false;