GrGLBlend::AppendPorterDuffBlend now supports kClear_Mode

src/gpu/gl/GrGLBlend.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 "GrGLBlend.h"
 #include "gl/builders/GrGLFragmentShaderBuilder.h"
 
 static bool append_porterduff_term(GrGLFragmentBuilder* fsBuilder, SkXfermode::Coeff coeff,
@@ skipping 40 matching lines @@
     }
 }
 
 void GrGLBlend::AppendPorterDuffBlend(GrGLFragmentBuilder* fsBuilder, const char* srcColor,
                                       const char* dstColor, const char* outColor,
                                       SkXfermode::Mode mode) {
 
     SkXfermode::Coeff srcCoeff, dstCoeff;
     SkXfermode::ModeAsCoeff(mode, &srcCoeff, &dstCoeff);
 
-    fsBuilder->codeAppendf("%s =", outColor);
+    fsBuilder->codeAppendf("%s = ", outColor);
     // append src blend
     bool didAppend = append_porterduff_term(fsBuilder, srcCoeff, srcColor, srcColor, dstColor,
                                             false);
     // append dst blend
-    SkAssertResult(append_porterduff_term(fsBuilder, dstCoeff, dstColor, srcColor, dstColor,
-                                          didAppend));
+    if(!append_porterduff_term(fsBuilder, dstCoeff, dstColor, srcColor, dstColor, didAppend)) {
+        fsBuilder->codeAppend("vec4(0, 0, 0, 0)");
+    }
     fsBuilder->codeAppend(";");
 }