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

include/gpu/effects/GrPorterDuffXferProcessor.h

Index: include/gpu/effects/GrPorterDuffXferProcessor.h
diff --git a/include/gpu/effects/GrPorterDuffXferProcessor.h b/include/gpu/effects/GrPorterDuffXferProcessor.h
index 1bd199370e2a22c27b1ea5a89821ccb7537d1d03..8d9e71fe1f3f50f6aa1e42c62e925f336586ff5d 100644
--- a/include/gpu/effects/GrPorterDuffXferProcessor.h
+++ b/include/gpu/effects/GrPorterDuffXferProcessor.h
@@ -28,8 +28,10 @@ public:
 
     bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const SK_OVERRIDE;
 
-    bool canApplyCoverage(const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const SK_OVERRIDE;
+    bool canApplyCoverage(const GrProcOptInfo& /*colorPOI*/,
+                          const GrProcOptInfo& /*coveragePOI*/) const SK_OVERRIDE {
+        return true;
+    }
 
     bool canTweakAlphaForCoverage() const SK_OVERRIDE;
 
@@ -39,11 +41,13 @@ public:
 private:
     GrPorterDuffXPFactory(GrBlendCoeff src, GrBlendCoeff dst); 
 
-    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;
 
     bool onIsEqual(const GrXPFactory& xpfBase) const SK_OVERRIDE {