Implement a computeFastBounds() traversal for SkImageFilter.

This allows for correct culling of primitives which have image filters applied.

R=reed@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/137423005

git-svn-id: http://skia.googlecode.com/svn/trunk@13207 2bbb7eff-a529-9590-31e7-b0007b416f81
16 files changed