discard when coverage is <= 0 in XP dst copy

src/gpu/gl/GrGLXferProcessor.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 "gl/GrGLXferProcessor.h"
 
 #include "GrXferProcessor.h"
 #include "gl/builders/GrGLFragmentShaderBuilder.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 
 void GrGLXferProcessor::emitCode(const EmitArgs& args) {
     if (args.fXP.getDstCopyTexture()) {
         bool topDown = kTopLeft_GrSurfaceOrigin == args.fXP.getDstCopyTexture()->origin();
 
         GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+
+        // We don't think any shaders actually output negative coverage, but just as a safety check
+        // for floating point precision errors we compare with <= here
+        fsBuilder->codeAppendf("if (all(lessThanEqual(%s, vec4(0)))) {"
+                               "    discard;"
+                               "}", args.fInputCoverage);
         const char* dstColor = fsBuilder->dstColor();
 
         const char* dstCopyTopLeftName;
         const char* dstCopyCoordScaleName;
 
         fDstCopyTopLeftUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
                                                    kVec2f_GrSLType,
                                                    kDefault_GrSLPrecision,
                                                    "DstCopyUpperLeft",
                                                    &dstCopyTopLeftName);
@@ skipping 30 matching lines @@
         } else {
             SkASSERT(!fDstCopyScaleUni.isValid());
         }
     } else {
         SkASSERT(!fDstCopyTopLeftUni.isValid());
         SkASSERT(!fDstCopyScaleUni.isValid());
     }
     this->onSetData(pdm, xp);
 }