Use dst copies in porter duffer XP to correctly render certain blends.

src/gpu/effects/GrCoverageSetOpXP.cpp

Index: src/gpu/effects/GrCoverageSetOpXP.cpp
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index f0ec1f9b00b4d7c74ce4431cb6dd551ab54c811e..40998d50336dd3f58e2b4e36798fffb46b7183c8 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -228,7 +228,8 @@ GrXPFactory* GrCoverageSetOpXPFactory::Create(SkRegion::Op regionOp, bool invert
 }
 
 GrXferProcessor*
-GrCoverageSetOpXPFactory::onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+GrCoverageSetOpXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                                const GrProcOptInfo& colorPOI,
                                                 const GrProcOptInfo& covPOI,
                                                 const GrDeviceCoordTexture* dstCopy) const {
     return CoverageSetOpXP::Create(fRegionOp, fInvertCoverage);