Use texture barriers to read directly from the RT

src/gpu/effects/GrPorterDuffXferProcessor.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrPorterDuffXferProcessor.h"
 
 #include "GrBlend.h"
@@ skipping 577 matching lines @@
         kRGBA_GrColorComponentFlags == knownColorFlags) {
         return true;
     }
     return false;
 }
 
 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 {
     if (!coveragePOI.isSolidWhite()) {
         output->fWillBlendWithDst = true;
         output->fBlendedColorFlags = 0;
         return;
     }
 
     GrBlendCoeff srcCoeff = fSrcCoeff;
@@ skipping 92 matching lines @@
 GR_DEFINE_XP_FACTORY_TEST(GrPorterDuffXPFactory);
 
 GrXPFactory* GrPorterDuffXPFactory::TestCreate(SkRandom* random,
                                                GrContext*,
                                                const GrDrawTargetCaps&,
                                                GrTexture*[]) {
     SkXfermode::Mode mode = SkXfermode::Mode(random->nextULessThan(SkXfermode::kLastCoeffMode));
     return GrPorterDuffXPFactory::Create(mode);
 }