Implement correct clipping for image filters.

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 37993c0ea3e2a527eafb2709061341c3e8fe86f3..e0439f201c1f45143b6e57d8918a857d7ea6ed39 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -83,7 +83,7 @@ public:
      *  Given the src bounds of an image, this returns the bounds of the result
      *  image after the filter has been applied.
      */
-    bool filterBounds(const SkIRect& src, const SkMatrix& ctm, SkIRect* dst);
+    bool filterBounds(const SkIRect& src, const SkMatrix& ctm, SkIRect* dst) const;
 
     /**
      *  Returns true if the filter can be processed on the GPU.  This is most
@@ -188,8 +188,9 @@ protected:
      */
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
                                SkBitmap* result, SkIPoint* offset);
-    // Default impl copies src into dst and returns true
-    virtual bool onFilterBounds(const SkIRect&, const SkMatrix&, SkIRect*);
+    // Default impl recursively unions all input bounds, or returns false
+    // if no inputs.

[reed1, 2014/02/04 15:30:08]

Perhaps augment dox:

- Given the bounds of the src image to be filtered (first parmeter) in device
coordinates, and the CTM, compute (conservatively) the resulting
device-coordinates bounds that this filter will produce. Used for culling and
temp-buffer allocations, so the result need not be exact, but should never be
smaller than the real answer.

[Stephen White, 2014/02/04 16:01:41]

Done, although I've changed the wording slightly. This one is not actually used
for
culling: computeFastBounds() is. This one applies the inverse transformation 
("what pixels of the source image would be required to fill the given 
destination rect").

+    virtual bool onFilterBounds(const SkIRect&, const SkMatrix&, SkIRect*) const;
 
     // Applies "matrix" to the crop rect, and sets "rect" to the intersection of
     // "rect" and the transformed crop rect. If there is no overlap, returns