Add gpu backend for SkPixelXorXfermode

src/gpu/glsl/GrGLSLXferProcessor.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "glsl/GrGLSLXferProcessor.h"
 
 #include "GrXferProcessor.h"
@@ skipping 70 matching lines @@
         } else {
             SkASSERT(!fDstScaleUni.isValid());
         }
     } else {
         SkASSERT(!fDstTopLeftUni.isValid());
         SkASSERT(!fDstScaleUni.isValid());
     }
     this->onSetData(pdm, xp);
 }
 
+void GrGLSLXferProcessor::DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
+                                                    const char* srcCoverage,
+                                                    const char* dstColor,
+                                                    const char* outColor,
+                                                    const char* outColorSecondary,
+                                                    const GrXferProcessor& proc) {
+    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);
+    }
+}