Use texture barriers to read directly from the RT

src/effects/SkArithmeticMode_gpu.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkArithmeticMode_gpu_DEFINED
 #define SkArithmeticMode_gpu_DEFINED
 
@@ skipping 67 matching lines @@
     }
 
     bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
         return true;
     }
 
     bool canTweakAlphaForCoverage() const override {
         return false;
     }
 
+    bool willBlendCoherently(const GrDrawTargetCaps& caps) const override {
+        return true;
+    }
+
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
     GrArithmeticXPFactory(float k1, float k2, float k3, float k4, bool enforcePMColor); 
 
     GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const override; 
@@ skipping 19 matching lines @@
     GR_DECLARE_XP_FACTORY_TEST;
 
     float                       fK1, fK2, fK3, fK4;
     bool                        fEnforcePMColor;
 
     typedef GrXPFactory INHERITED;
 };
 
 #endif
 #endif