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

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/997c6358d94e188b1a7b89a4f86e24cbe0f5a164

Review URL: https://codereview.chromium.org/914003003
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index 2b26211..6f49a1d 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -223,7 +223,8 @@
 }
 
 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);