discard when coverage is <= 0 in XP dst copy.

Darken was 5-20% slower with this CL, though it might improve things a bit to only look at one component of the vector.

BUG=skia:

Review URL: https://codereview.chromium.org/915633002

src/gpu/gl/GrGLXferProcessor.cpp

@@ -16,6 +16,12 @@ void GrGLXferProcessor::emitCode(const EmitArgs& args) {
         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;