Add mixed samples support to XPs

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 3d3ab49dadc89ba64e0c84c40ca21284286af26a..e62497193fbcc38a39c42b2138ee0fe3ca50e26c 100644
--- a/src/gpu/gl/GrGLXferProcessor.cpp
+++ b/src/gpu/gl/GrGLXferProcessor.cpp
@@ -63,7 +63,14 @@ void GrGLXferProcessor::emitCode(const EmitArgs& args) {
                                   args.fXP);
 
     // Apply coverage.
-    if (args.fXP.readsCoverage()) {
+    if (args.fXP.dstReadUsesMixedSamples()) {
+        if (args.fXP.readsCoverage()) {
+            fsBuilder->codeAppendf("%s *= %s;", args.fOutputPrimary, args.fInputCoverage);
+            fsBuilder->codeAppendf("%s = %s;", args.fOutputSecondary, args.fInputCoverage);
+        } else {
+            fsBuilder->codeAppendf("%s = vec4(1.0);", args.fOutputSecondary);
+        }
+    } else if (args.fXP.readsCoverage()) {
         fsBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
                                args.fOutputPrimary, args.fInputCoverage,
                                args.fOutputPrimary, args.fInputCoverage, dstColor);