Bounds fixes for SkXfermodeImageFilter:

Bounds fixes for SkXfermodeImageFilter:

1) Change the default bounds to the union of the foreground and background bounds.
2) Use a canvas translate instead of manually offsetting the foreground and background bounds by the union.
3) Apply the transfer mode to all pixels, including those outside the foreground rect by using a difference clip.

Covered by the offset test cases in the xfermodeimagefilter GM (will need rebaselines).

R=reed@google.com

Committed: https://code.google.com/p/skia/source/detail?r=12652

[Stephen White, 2013-12-12 15:34:51]

reed@: PTAL. Thanks!

[reed1, 2013-12-12 16:32:53]

lgtm

[commit-bot: I haz the power, 2013-12-12 17:12:08]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/senorblanco@chromium.org/112683004/20001

[Stephen White, 2013-12-12 21:01:04]

Committed patchset #2 manually as r12652 (presubmit successful).