Don't create a GXPFactory when blend is SrcOver

include/gpu/effects/GrPorterDuffXferProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrPorterDuffXferProcessor_DEFINED
 #define GrPorterDuffXferProcessor_DEFINED
 
 #include "GrTypes.h"
 #include "GrXferProcessor.h"
 #include "SkXfermode.h"
 
 class GrProcOptInfo;
 
 class GrPorterDuffXPFactory : public GrXPFactory {
 public:
     static GrXPFactory* Create(SkXfermode::Mode mode); 
 
-    bool supportsRGBCoverage(GrColor /*knownColor*/, uint32_t /*knownColorFlags*/) const override {
-        return true;
-    }
-
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;
 
+    static GrXferProcessor* CreateSrcOverXferProcessor(const GrCaps& caps,
+                                                       const GrProcOptInfo& colorPOI,
+                                                       const GrProcOptInfo& coveragePOI,
+                                                       bool hasMixedSamples,
+                                                       const GrXferProcessor::DstTexture*);
+
+    static inline void SrcOverInvariantBlendedColor(
+                                                GrColor inputColor,
+                                                GrColorComponentFlags validColorFlags,
+                                                bool isOpaque,
+                                                GrXPFactory::InvariantBlendedColor* blendedColor) {
+        if (!isOpaque) {
+            blendedColor->fWillBlendWithDst = true;
+            blendedColor->fKnownColorFlags = kNone_GrColorComponentFlags;
+            return;
+        }
+        blendedColor->fWillBlendWithDst = false;
+
+        blendedColor->fKnownColor = inputColor;
+        blendedColor->fKnownColorFlags = validColorFlags;
+    }
+
+    static bool SrcOverWillNeedDstTexture(const GrCaps& caps,
+                                          const GrProcOptInfo& colorPOI,
+                                          const GrProcOptInfo& coveragePOI,
+                                          bool hasMixedSamples);
+
 private:
     GrPorterDuffXPFactory(SkXfermode::Mode);
 
     GrXferProcessor* onCreateXferProcessor(const GrCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            bool hasMixedSamples,
                                            const DstTexture*) const override;
 
     bool willReadDstColor(const GrCaps& caps,
                           const GrProcOptInfo& colorPOI,
                           const GrProcOptInfo& coveragePOI,
                           bool hasMixedSamples) const override;
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrPorterDuffXPFactory& xpf = xpfBase.cast<GrPorterDuffXPFactory>();
         return fXfermode == xpf.fXfermode;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
     static void TestGetXPOutputTypes(const GrXferProcessor*, int* outPrimary, int* outSecondary);
 
     SkXfermode::Mode fXfermode;
 
     friend class GrPorterDuffTest; // for TestGetXPOutputTypes()
     typedef GrXPFactory INHERITED;
 };
 
 #endif