Only discard for XP dstCopies if we have a coverage value.

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 c517381249ae77735346c5a050d026ab38467e20..657da41274af0d8f3a7e19c559340d79d7afa668 100644
--- a/src/gpu/gl/GrGLXferProcessor.cpp
+++ b/src/gpu/gl/GrGLXferProcessor.cpp
@@ -17,11 +17,14 @@ void GrGLXferProcessor::emitCode(const EmitArgs& args) {
 
         GrGLXPFragmentBuilder* 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);
+        if (args.fXP.readsCoverage()) {
+            // 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;