Add gpu backend for SkPixelXorXfermode

src/gpu/glsl/GrGLSLXferProcessor.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLSLXferProcessor_DEFINED
 #define GrGLSLXferProcessor_DEFINED
 
@@ skipping 50 matching lines @@
 
     /** A GrGLSLXferProcessor instance can be reused with any GrGLSLXferProcessor that produces
         the same stage key; this function reads data from a GrGLSLXferProcessor and uploads any
         uniform variables required  by the shaders created in emitCode(). The GrXferProcessor
         parameter is guaranteed to be of the same type that created this GrGLSLXferProcessor and
         to have an identical processor key as the one that created this GrGLSLXferProcessor. This
         function calls onSetData on the subclass of GrGLSLXferProcessor
      */
     void setData(const GrGLSLProgramDataManager& pdm, const GrXferProcessor& xp);
 
+protected:
+    static void DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
+                                          const char* srcCoverage,
+                                          const char* dstColor,
+                                          const char* outColor,
+                                          const char* outColorSecondary,
+                                          const GrXferProcessor& proc);
+    
 private:
     /**
      * Called by emitCode() when the XP will not be performing a dst read. This method is
      * responsible for both blending and coverage. A subclass only needs to implement this method if
      * it can construct a GrXferProcessor that will not read the dst color.
      */
     virtual void emitOutputsForBlendState(const EmitArgs&) {
         SkFAIL("emitOutputsForBlendState not implemented.");
     }
 
@@ skipping 12 matching lines @@
                                          const GrXferProcessor&) {
         SkFAIL("emitBlendCodeForDstRead not implemented.");
     }
 
     virtual void onSetData(const GrGLSLProgramDataManager&, const GrXferProcessor&) = 0;
 
     GrGLSLProgramDataManager::UniformHandle fDstTopLeftUni;
     GrGLSLProgramDataManager::UniformHandle fDstScaleUni;
 };
 #endif