Add a method of returning the list of commands that use an image

Change-Id: I3c94f7b6faa71ab0cbeccddd2032b657e350c9b3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316843
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
6 files changed