add ways to peer into an image to get its pixels

BUG=skia:
R=bsalomon@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/155763004

git-svn-id: http://skia.googlecode.com/svn/trunk@13339 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed