Use texture barriers to read directly from the RT

src/gpu/effects/GrPorterDuffXferProcessor.cpp

Index: src/gpu/effects/GrPorterDuffXferProcessor.cpp
diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 77b7fcaac00c5e52a6d433124d513ea5ca8f208d..51c3c765bac13c6d2d2168bd47a2b9455370f80e 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -595,6 +595,10 @@ bool GrPorterDuffXPFactory::canTweakAlphaForCoverage() const {
     return can_tweak_alpha_for_coverage(fDstCoeff);
 }
 
+bool GrPorterDuffXPFactory::willBlendCoherently(const GrDrawTargetCaps& caps) const {
+    return true;
+}
+
 void GrPorterDuffXPFactory::getInvariantOutput(const GrProcOptInfo& colorPOI,
                                                const GrProcOptInfo& coveragePOI,
                                                GrXPFactory::InvariantOutput* output) const {