move SkImageInfo into its own header

BUG=
R=djsollen@google.com

Review URL: https://codereview.chromium.org/71813002

git-svn-id: http://skia.googlecode.com/svn/trunk/src@12273 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed