Image filters: implement offsets in GPU path.

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 78e321a7b5693307224cfee271470098ba5efbb2..f29b4606b5331b433cafd9fbaa14902f4917273d 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -109,10 +109,11 @@ public:
      *  textures.  For single-pass effects, use asNewEffect().  src is the
      *  source image for processing, as a texture-backed bitmap.  result is
      *  the destination bitmap, which should contain a texture-backed pixelref
-     *  on success.  The default implementation does single-pass processing
-     *  using asNewEffect().
+     *  on success.  offset is the amount to translate the resulting image 
+     *  relative to the src when it is drawn. The default implementation does
+     *  single-pass processing using asNewEffect().
      */
-    virtual bool filterImageGPU(Proxy*, const SkBitmap& src, SkBitmap* result);
+    virtual bool filterImageGPU(Proxy*, const SkBitmap& src, SkBitmap* result, SkIPoint* offset);
 
     /**
      *  Returns whether this image filter is a color filter and puts the color filter into the