Use dst copies in porter duffer XP to correctly render certain blends.

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 78 matching lines @@
     bool canTweakAlphaForCoverage() const SK_OVERRIDE {
         return false;
     }
 
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const SK_OVERRIDE;
 
 private:
     GrArithmeticXPFactory(float k1, float k2, float k3, float k4, bool enforcePMColor); 
 
-    GrXferProcessor* onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+    GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                           const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const SK_OVERRIDE; 
 
-    bool willReadDstColor(const GrProcOptInfo& colorPOI,
+    bool willReadDstColor(const GrDrawTargetCaps& caps,
+                          const GrProcOptInfo& colorPOI,
                           const GrProcOptInfo& coveragePOI) const SK_OVERRIDE {
         return true;
     }
 
     bool onIsEqual(const GrXPFactory& xpfBase) const SK_OVERRIDE {
         const GrArithmeticXPFactory& xpf = xpfBase.cast<GrArithmeticXPFactory>();
         if (fK1 != xpf.fK1 ||
             fK2 != xpf.fK2 ||
             fK3 != xpf.fK3 ||
             fK4 != xpf.fK4 ||
             fEnforcePMColor != xpf.fEnforcePMColor) {
             return false;
         }
         return true;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     float                       fK1, fK2, fK3, fK4;
     bool                        fEnforcePMColor;
 
     typedef GrXPFactory INHERITED;
 };
 
 #endif
 #endif