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 216fb62d43b164e74b1734f6d774e5fccffe34cb..2ac8591edad91091c21b01d407bdf78c1d76eca3 100644
--- a/src/gpu/effects/GrCustomXfermodePriv.h
+++ b/src/gpu/effects/GrCustomXfermodePriv.h
@@ -64,9 +64,7 @@ public:
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override {
-        return false;
-    }
+    bool canTweakAlphaForCoverage() const override;
 
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;
@@ -79,9 +77,7 @@ 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>();