Some upcoming work (support for expanding crop rects) requires
the clip bounds to be available during filter traversal. This change
replaces the SkMatrix parameter in the onFilterImage() traversals
with a Context parameter. It contains the CTM, as well as the clip
bounds.
BUG=skia:
R=reed@google.com
Review URL: https://codereview.chromium.org/189913021
git-svn-id: http://skia.googlecode.com/svn/trunk@13803 2bbb7eff-a529-9590-31e7-b0007b416f81