Use texture barriers to read directly from the RT

include/gpu/effects/GrCoverageSetOpXP.h

Index: include/gpu/effects/GrCoverageSetOpXP.h
diff --git a/include/gpu/effects/GrCoverageSetOpXP.h b/include/gpu/effects/GrCoverageSetOpXP.h
index 64002d5b9762774f78d31d38aec3c51be76b4662..1c8e67f2ee3cdc5b135b682a370c1ecbf03be820 100644
--- a/include/gpu/effects/GrCoverageSetOpXP.h
+++ b/include/gpu/effects/GrCoverageSetOpXP.h
@@ -30,6 +30,8 @@ public:
 
     bool canTweakAlphaForCoverage() const override { return false; }
 
+    bool willBlendCoherently(const GrDrawTargetCaps&) const override { return true; }
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;