Add gpu implementation of OverdrawXfermode

src/effects/SkArithmeticMode_gpu.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 "SkArithmeticMode_gpu.h"
 
 #if SK_SUPPORT_GPU
@@ skipping 208 matching lines @@
                                  const char* outColorSecondary,
                                  const GrXferProcessor& proc) override {
         fKUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
                                            kVec4f_GrSLType, kDefault_GrSLPrecision,
                                            "k");
         const char* kUni = uniformHandler->getUniformCStr(fKUni);
 
         add_arithmetic_code(fragBuilder, srcColor, dstColor, outColor, kUni, fEnforcePMColor);
 
         // Apply coverage.
-        if (proc.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, proc);
     }
 
     void onSetData(const GrGLSLProgramDataManager& pdman,
                    const GrXferProcessor& processor) override {
         const ArithmeticXP& arith = processor.cast<ArithmeticXP>();
         pdman.set4f(fKUni, arith.k1(), arith.k2(), arith.k3(), arith.k4());
         fEnforcePMColor = arith.enforcePMColor();
     };
 
     GrGLSLProgramDataManager::UniformHandle fKUni;
@@ skipping 62 matching lines @@
     float k1 = d->fRandom->nextF();
     float k2 = d->fRandom->nextF();
     float k3 = d->fRandom->nextF();
     float k4 = d->fRandom->nextF();
     bool enforcePMColor = d->fRandom->nextBool();
 
     return GrArithmeticXPFactory::Create(k1, k2, k3, k4, enforcePMColor);
 }
 
 #endif