Revert of Make specialized SrcOver XPFactory

src/gpu/SkGr.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkGr.h"
 
@@ skipping 470 matching lines @@
                 colorFilter->asFragmentProcessor(context));
             if (cfFP) {
                 grPaint->addColorFragmentProcessor(cfFP);
             } else {
                 return false;
             }
         }
     }
 
     SkXfermode* mode = skPaint.getXfermode();
-    const GrXPFactory* xpFactory = nullptr;
+    GrXPFactory* xpFactory = nullptr;
     if (!SkXfermode::AsXPFactory(mode, &xpFactory)) {
         // Fall back to src-over
         // return false here?
         xpFactory = GrPorterDuffXPFactory::Create(SkXfermode::kSrcOver_Mode);
     }
     SkASSERT(xpFactory);
     grPaint->setXPFactory(xpFactory)->unref();
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {
@@ skipping 130 matching lines @@
             SkErrorInternals::SetError( kInvalidPaint_SkError,
                                         "Sorry, I don't understand the filtering "
                                         "mode you asked for.  Falling back to "
                                         "MIPMaps.");
             textureFilterMode = GrTextureParams::kMipMap_FilterMode;
             break;
 
     }
     return textureFilterMode;
 }