Use texture barriers to read directly from the RT

src/gpu/effects/GrDisableColorXP.h

Index: src/gpu/effects/GrDisableColorXP.h
diff --git a/src/gpu/effects/GrDisableColorXP.h b/src/gpu/effects/GrDisableColorXP.h
index 9bbe24d757c10f0d0a313e7485afa95dfbaea50f..e17e1d13137d9d623116a7245a467dfbfd8c38a6 100644
--- a/src/gpu/effects/GrDisableColorXP.h
+++ b/src/gpu/effects/GrDisableColorXP.h
@@ -25,6 +25,8 @@ public:
 
     bool canTweakAlphaForCoverage() const override { return true; }
 
+    bool willBlendCoherently(const GrDrawTargetCaps&) const override { return true; }
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput* output) const override {
         output->fBlendedColorFlags = 0;