Use texture barriers to read directly from the RT

include/gpu/effects/GrPorterDuffXferProcessor.h

Index: include/gpu/effects/GrPorterDuffXferProcessor.h
diff --git a/include/gpu/effects/GrPorterDuffXferProcessor.h b/include/gpu/effects/GrPorterDuffXferProcessor.h
index 65d7a723f6696573e512a74ddd87ec9dc7f5bca0..f600a5abadddd29b06b49232346603ed9e67d8e3 100644
--- a/include/gpu/effects/GrPorterDuffXferProcessor.h
+++ b/include/gpu/effects/GrPorterDuffXferProcessor.h
@@ -22,6 +22,8 @@ public:
 
     bool canTweakAlphaForCoverage() const override;
 
+    bool willBlendCoherently(const GrDrawTargetCaps& caps) const override;
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;