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

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
===================================================================
--- src/gpu/GrGpu.h	(revision 8448)
+++ src/gpu/GrGpu.h	(working copy)
@@ -366,7 +366,7 @@
     }
 
     // prepares clip flushes gpu state before a draw
-    bool setupClipAndFlushState(DrawType);
+    bool setupClipAndFlushState(DrawType, const GrDeviceCoordTexture* dstCopy);
 
     // Functions used to map clip-respecting stencil tests into normal
     // stencil funcs supported by GPUs.
@@ -482,9 +482,9 @@
 
     // The GrGpu typically records the clients requested state and then flushes
     // deltas from previous state at draw time. This function does the
-    // backend-specific flush of the state
+    // backend-specific flush of the state.
     // returns false if current state is unsupported.
-    virtual bool flushGraphicsState(DrawType) = 0;
+    virtual bool flushGraphicsState(DrawType, const GrDeviceCoordTexture* dstCopy) = 0;
 
     // clears the entire stencil buffer to 0
     virtual void clearStencil() = 0;