discard when coverage is <= 0 in XP dst copy

src/gpu/gl/GrGLXferProcessor.cpp

Index: src/gpu/gl/GrGLXferProcessor.cpp
diff --git a/src/gpu/gl/GrGLXferProcessor.cpp b/src/gpu/gl/GrGLXferProcessor.cpp
index 4aac11d3354c53533e0ec128f1be55488078dd1b..b7bf3019b5921c3c5aca4d0aa2234f1a96f7f94f 100644
--- a/src/gpu/gl/GrGLXferProcessor.cpp
+++ b/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;