Add caching of the snapshot image form a surface

Add caching of the snapshot image form a surface
Notify the surface when the canvas draws into it, so it can invalidate the
cached image, and (if needed) perform a copy-on-write on the surface if it
was being shared with the image.
Review URL: https://codereview.appspot.com/6441115

git-svn-id: http://skia.googlecode.com/svn/trunk@5306 2bbb7eff-a529-9590-31e7-b0007b416f81