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

src/effects/SkArithmeticMode_gpu.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkArithmeticMode_gpu.h"
 
 #if SK_SUPPORT_GPU
@@ skipping 286 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
 GrArithmeticXPFactory::GrArithmeticXPFactory(float k1, float k2, float k3, float k4,
                                              bool enforcePMColor) 
     : fK1(k1), fK2(k2), fK3(k3), fK4(k4), fEnforcePMColor(enforcePMColor) {
     this->initClassID<GrArithmeticXPFactory>();
 }
 
 GrXferProcessor*
-GrArithmeticXPFactory::onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+GrArithmeticXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                             const GrProcOptInfo& colorPOI,
                                              const GrProcOptInfo& coveragePOI,
                                              const GrDeviceCoordTexture* dstCopy) const {
     return ArithmeticXP::Create(fK1, fK2, fK3, fK4, fEnforcePMColor, dstCopy,
-                                this->willReadDstColor(colorPOI, coveragePOI));
+                                this->willReadDstColor(caps, colorPOI, coveragePOI));
 }
 
 
 void GrArithmeticXPFactory::getInvariantOutput(const GrProcOptInfo& colorPOI,
                                                const GrProcOptInfo& coveragePOI,
                                                GrXPFactory::InvariantOutput* output) const {
     output->fWillBlendWithDst = true;
 
     // TODO: We could try to optimize this more. For example if we have solid coverage and fK1 and
     // fK3 are zero, then we won't be blending the color with dst at all so we can know what the
@@ skipping 10 matching lines @@
     float k1 = random->nextF();
     float k2 = random->nextF();
     float k3 = random->nextF();
     float k4 = random->nextF();
     bool enforcePMColor = random->nextBool();
 
     return GrArithmeticXPFactory::Create(k1, k2, k3, k4, enforcePMColor);
 }
 
 #endif