Don't create a GXPFactory when blend is SrcOver

src/effects/SkDisplacementMapEffect.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDisplacementMapEffect.h"
 #include "SkDevice.h"
 #include "SkReadBuffer.h"
@@ skipping 427 matching lines @@
                               SkIntToScalar(colorOffset.fY - displacementOffset.fY));
 
     paint.addColorFragmentProcessor(
         GrDisplacementMapEffect::Create(fXChannelSelector,
                                         fYChannelSelector,
                                         scale,
                                         displacement,
                                         offsetMatrix,
                                         color,
                                         colorBM.dimensions()))->unref();
+    paint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
     SkIRect colorBounds = bounds;
     colorBounds.offset(-colorOffset);
     SkMatrix matrix;
     matrix.setTranslate(-SkIntToScalar(colorBounds.x()),
                         -SkIntToScalar(colorBounds.y()));
 
     SkAutoTUnref<GrDrawContext> drawContext(context->drawContext(dst->asRenderTarget()));
     if (!drawContext) {
         return false;
     }
@@ skipping 175 matching lines @@
                                        const GrGLSLCaps&, GrProcessorKeyBuilder* b) {
     const GrDisplacementMapEffect& displacementMap = proc.cast<GrDisplacementMapEffect>();
 
     uint32_t xKey = displacementMap.xChannelSelector();
     uint32_t yKey = displacementMap.yChannelSelector() << kChannelSelectorKeyBits;
 
     b->add32(xKey | yKey);
 }
 #endif