Implement support for a Context parameter in image filters

Implement support for a Context parameter in image filters

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