Add gpu implementation of OverdrawXfermode

src/gpu/effects/GrCustomXfermode.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrCustomXfermode.h"
 
 #include "GrCoordTransform.h"
@@ skipping 154 matching lines @@
                                  const char* dstColor,
                                  const char* outColor,
                                  const char* outColorSecondary,
                                  const GrXferProcessor& proc) override {
         const CustomXP& xp = proc.cast<CustomXP>();
         SkASSERT(!xp.hasHWBlendEquation());
 
         GrGLSLBlend::AppendMode(fragBuilder, srcColor, dstColor, outColor, xp.mode());
 
         // Apply coverage.
-        if (xp.dstReadUsesMixedSamples()) {
-            if (srcCoverage) {
-                fragBuilder->codeAppendf("%s *= %s;", outColor, srcCoverage);
-                fragBuilder->codeAppendf("%s = %s;", outColorSecondary, srcCoverage);
-            } else {
-                fragBuilder->codeAppendf("%s = vec4(1.0);", outColorSecondary);
-            }
-        } else if (srcCoverage) {
-            fragBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
-                                     outColor, srcCoverage, outColor, srcCoverage, dstColor);
-        }
+        INHERITED::DefaultCoverageModulation(fragBuilder, srcCoverage, dstColor, outColor,
+                                             outColorSecondary, xp);
     }
 
     void onSetData(const GrGLSLProgramDataManager&, const GrXferProcessor&) override {}
 
     typedef GrGLSLXferProcessor INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 void CustomXP::onGetGLSLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const {
@@ skipping 204 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
 GrXPFactory* GrCustomXfermode::CreateXPFactory(SkXfermode::Mode mode) {
     if (!GrCustomXfermode::IsSupportedMode(mode)) {
         return nullptr;
     } else {
         return new CustomXPFactory(mode);
     }
 }