add encodeData() to SkImageEncoder, and add encoding to SkImage

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/15002004

git-svn-id: http://skia.googlecode.com/svn/trunk@9193 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed