Add onGetBlendInfo to GrXferProcessor

include/gpu/GrXferProcessor.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 GrXferProcessor_DEFINED
 #define GrXferProcessor_DEFINED
 
@@ skipping 107 matching lines @@
      * A caller who calls this function on a XP is required to honor the returned OptFlags
      * and color values for its draw.
      */
     virtual OptFlags getOptimizations(const GrProcOptInfo& colorPOI,
                                       const GrProcOptInfo& coveragePOI,
                                       bool doesStencilWrite,
                                       GrColor* overrideColor,
                                       const GrDrawTargetCaps& caps) = 0;
 
     struct BlendInfo {
-        BlendInfo() : fWriteColor(true) {}
+        void reset() {
+            fSrcBlend = kOne_GrBlendCoeff;
+            fDstBlend = kZero_GrBlendCoeff;
+            fBlendConstant = 0;
+            fWriteColor = true;
+        }
 
         GrBlendCoeff fSrcBlend;
         GrBlendCoeff fDstBlend;
         GrColor      fBlendConstant;
         bool         fWriteColor;
     };
 
-    virtual void getBlendInfo(BlendInfo* blendInfo) const = 0;
+    void getBlendInfo(BlendInfo* blendInfo) const {
+        blendInfo->reset();
+        this->onGetBlendInfo(blendInfo);
+    }
 
     bool willReadDstColor() const { return fWillReadDstColor; }
 
     /**
      * Returns the texture to be used as the destination when reading the dst in the fragment
      * shader. If the returned texture is NULL then the XP is either not reading the dst or we have
      * extentions that support framebuffer fetching and thus don't need a copy of the dst texture.
      */
     const GrTexture* getDstCopyTexture() const { return fDstCopy.getTexture(); }
 
@@ skipping 40 matching lines @@
     GrXferProcessor(const GrDeviceCoordTexture* dstCopy, bool willReadDstColor);
 
 private:
     /**
      * Sets a unique key on the GrProcessorKeyBuilder that is directly associated with this xfer
      * processor's GL backend implementation.
      */
     virtual void onGetGLProcessorKey(const GrGLCaps& caps,
                                      GrProcessorKeyBuilder* b) const = 0;
 
+    /**
+     * Retrieves the hardware blend state required by this Xfer processor. The BlendInfo struct
+     * comes initialized to default values, so the Xfer processor only needs to set the state it
+     * needs. It may not even need to override this method at all.
+     */
+    virtual void onGetBlendInfo(BlendInfo*) const {}
+
     virtual bool onIsEqual(const GrXferProcessor&) const = 0;
 
     bool                    fWillReadDstColor;
     SkIPoint                fDstCopyTextureOffset;
     GrTextureAccess         fDstCopy;
 
     typedef GrFragmentProcessor INHERITED;
 };
 
 GR_MAKE_BITFIELD_OPS(GrXferProcessor::OptFlags);
@@ skipping 103 matching lines @@
     enum {
         kIllegalXPFClassID = 0,
     };
     static int32_t gCurrXPFClassID;
 
     typedef GrProgramElement INHERITED;
 };
 
 #endif