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

src/gpu/effects/GrCustomXfermode.cpp

Index: src/gpu/effects/GrCustomXfermode.cpp
diff --git a/src/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index 8d8f9f251d686b5d3c217341bf44863b3a18f40b..e4d65e41e46c3f78d821723917bf71966f3574e7 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -608,10 +608,11 @@ GrCustomXPFactory::GrCustomXPFactory(SkXfermode::Mode mode)
 }
 
 GrXferProcessor*
-GrCustomXPFactory::onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+GrCustomXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                         const GrProcOptInfo& colorPOI,
                                          const GrProcOptInfo& coveragePOI,
                                          const GrDeviceCoordTexture* dstCopy) const {
-    return CustomXP::Create(fMode, dstCopy, this->willReadDstColor(colorPOI, coveragePOI));
+    return CustomXP::Create(fMode, dstCopy, this->willReadDstColor(caps, colorPOI, coveragePOI));
 }