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

Review URL: https://codereview.chromium.org/112683004

git-svn-id: http://skia.googlecode.com/svn/trunk@12652 2bbb7eff-a529-9590-31e7-b0007b416f81