Perform coverage blend with the dst in the shader when using a dst-reading xfermode.

src/gpu/SkGpuDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGpuDevice.h"
 
 #include "effects/GrTextureDomainEffect.h"
@@ skipping 483 matching lines @@
     grPaint->setAntiAlias(skPaint.isAntiAlias());
 
     SkXfermode::Coeff sm;
     SkXfermode::Coeff dm;
 
     SkXfermode* mode = skPaint.getXfermode();
     GrEffectRef* xferEffect = NULL;
     if (SkXfermode::AsNewEffectOrCoeff(mode, dev->context(), &xferEffect, &sm, &dm)) {
         if (NULL != xferEffect) {
             grPaint->colorStage(kXfermodeEffectIdx)->setEffect(xferEffect)->unref();
-            // This may not be the right place to have this logic but we set the GPU blend to
-            // src-over so that fractional coverage will be accounted for correctly.
             sm = SkXfermode::kOne_Coeff;
-            dm = SkXfermode::kISA_Coeff;
+            dm = SkXfermode::kZero_Coeff;
         }
     } else {
         //SkDEBUGCODE(SkDebugf("Unsupported xfer mode.\n");)
 #if 0
         return false;
 #else
         // Fall back to src-over
         sm = SkXfermode::kOne_Coeff;
         dm = SkXfermode::kISA_Coeff;
 #endif
@@ skipping 1308 matching lines @@
                          GrTexture* texture,
                          bool needClear)
     : SkDevice(make_bitmap(context, texture->asRenderTarget())) {
 
     GrAssert(texture && texture->asRenderTarget());
     // This constructor is called from onCreateCompatibleDevice. It has locked the RT in the texture
     // cache. We pass true for the third argument so that it will get unlocked.
     this->initFromRenderTarget(context, texture->asRenderTarget(), true);
     fNeedClear = needClear;
 }