Add support for reading the dst pixel value in an effect. Use in a new effect for the kDarken xfer …

src/gpu/gl/GrGpuGL.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 
 #ifndef GrGpuGL_DEFINED
@@ skipping 117 matching lines @@
 
     virtual void setStencilPathSettings(const GrPath&,
                                         SkPath::FillType,
                                         GrStencilSettings* settings)
                                         SK_OVERRIDE;
     virtual void onGpuStencilPath(const GrPath*, SkPath::FillType) SK_OVERRIDE;
 
     virtual void clearStencil() SK_OVERRIDE;
     virtual void clearStencilClip(const GrIRect& rect,
                                   bool insideClip) SK_OVERRIDE;
-    virtual bool flushGraphicsState(DrawType) SK_OVERRIDE;
+    virtual bool flushGraphicsState(DrawType, const GrDeviceCoordTexture* dstCopy) SK_OVERRIDE;
 
     // binds texture unit in GL
     void setTextureUnit(int unitIdx);
 
     // Sets up vertex attribute pointers and strides. On return indexOffsetInBytes gives the offset
     // an into the index buffer. It does not account for drawInfo.startIndex() but rather the start
     // index is relative to the returned offset.
     void setupGeometry(const DrawInfo& info, size_t* indexOffsetInBytes);
 
     // Subclasses should call this to flush the blend state.
@@ skipping 276 matching lines @@
     ///@}
 
     // we record what stencil format worked last time to hopefully exit early
     // from our loop that tries stencil formats and calls check fb status.
     int fLastSuccessfulStencilFmtIdx;
 
     typedef GrGpu INHERITED;
 };
 
 #endif