Implement support for KHR_blend_equation_advanced

src/gpu/effects/GrCustomXfermodePriv.h

Index: src/gpu/effects/GrCustomXfermodePriv.h
diff --git a/src/gpu/effects/GrCustomXfermodePriv.h b/src/gpu/effects/GrCustomXfermodePriv.h
index edcdcc0a6e014f33ba8009c0860884c4b7849153..43aaeabc54992cb9a5bc1ce4751a20fa49e93c25 100644
--- a/src/gpu/effects/GrCustomXfermodePriv.h
+++ b/src/gpu/effects/GrCustomXfermodePriv.h
@@ -75,9 +75,7 @@ private:
 
     bool willReadDstColor(const GrDrawTargetCaps& caps,
                           const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const override {
-        return true;
-    }
+                          const GrProcOptInfo& coveragePOI) const override;
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrCustomXPFactory& xpf = xpfBase.cast<GrCustomXPFactory>();