DM: find max component difference

So far unused except to back BitmapsEqual, but I want to play with it in CLs I'm working on.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/290043005

git-svn-id: http://skia.googlecode.com/svn/trunk@14759 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed