Add gpu implementation of SkAvoidXfermode

gm/avoidxfermode.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 49 matching lines @@
         SkPaint p2;
         p2.setColor(SK_ColorBLACK);
         p2.setXfermode(SkAvoidXfermode::Create(SK_ColorWHITE,
                                                250,
                                                SkAvoidXfermode::kAvoidColor_Mode))->unref();
 
         canvas->drawRect(r, p2);
 
         r.offsetTo(0.0f, 64);
 
-        // LL corner: replace red with blue with a loose tolerance
+        // LL corner: replace red with transparent blue with a loose tolerance
         SkPaint p3;
-        p3.setColor(SK_ColorBLUE);
+        p3.setColor(0x7F0000FF);
         p3.setXfermode(SkAvoidXfermode::Create(SK_ColorRED,
                                                250,
                                                SkAvoidXfermode::kTargetColor_Mode))->unref();
 
         canvas->drawRect(r, p3);
 
         r.offsetTo(64, 64);
 
         // LR corner: draw black everywhere except red with a loose tolerance
         SkPaint p4;
         p4.setColor(SK_ColorBLACK);
         p4.setXfermode(SkAvoidXfermode::Create(SK_ColorRED,
                                                5,
                                                SkAvoidXfermode::kAvoidColor_Mode))->unref();
 
         canvas->drawRect(r, p4);
     }
 
 private:
     SkBitmap fBM;
 
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM(return new AvoidXfermodeGM;)