Add gpu backend for SkPixelXorXfermode

gm/pixelxorxfermode.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "Resources.h"
 
@@ skipping 31 matching lines @@
 
     void onDraw(SkCanvas* canvas) override {
         canvas->drawBitmap(fBM, 0, 0);
 
         SkRect r = SkRect::MakeIWH(256, 256);
 
         // Negate the red channel of the dst (via the ancillary color) but leave
         // the green & blue channels alone
         SkPaint p1;
         p1.setColor(SK_ColorBLACK); // noop
-        p1.setXfermode(SkPixelXorXfermode::Create(SK_ColorRED))->unref();
+        p1.setXfermode(SkPixelXorXfermode::Create(0x7FFF0000))->unref();
 
         canvas->drawRect(r, p1);
 
         r.offsetTo(256.0f, 0.0f);
 
         // Negate the dst color via the src color
         SkPaint p2;
         p2.setColor(SK_ColorWHITE);
         p2.setXfermode(SkPixelXorXfermode::Create(SK_ColorBLACK))->unref(); // noop
 
@@ skipping 21 matching lines @@
 
 private:
     SkBitmap fBM;
 
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM(return new PixelXorXfermodeGM;)