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

include/gpu/effects/GrCoverageSetOpXP.h

Index: include/gpu/effects/GrCoverageSetOpXP.h
diff --git a/include/gpu/effects/GrCoverageSetOpXP.h b/include/gpu/effects/GrCoverageSetOpXP.h
index e781999a1dfbc7153c8a8e925466c17d2839d974..a95326f4840ab5f34e23a2be66b38a7afcfae0cf 100644
--- a/include/gpu/effects/GrCoverageSetOpXP.h
+++ b/include/gpu/effects/GrCoverageSetOpXP.h
@@ -41,11 +41,13 @@ public:
 private:
     GrCoverageSetOpXPFactory(SkRegion::Op regionOp, bool invertCoverage);
 
-    GrXferProcessor* onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+    GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                           const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const SK_OVERRIDE;
 
-    bool willReadDstColor(const GrProcOptInfo& /*colorPOI*/,
+    bool willReadDstColor(const GrDrawTargetCaps& /*caps*/,
+                          const GrProcOptInfo& /*colorPOI*/,
                           const GrProcOptInfo& /*coveragePOI*/) const SK_OVERRIDE {
         return false;
     }