We should make image decode tasks cancellable:

- If we're processing an image decode, and we get a new schedule without that image decode, we can cancel the task
- We can plumb a bool/callback to the task to see if it was canceled
- The actual class doing the work can check this bool
- It can react by aborting future work, and ensure that if the task runs again things just work (tm)

This should allow us to cancel long running image decode tasks on a fast fling through image heavy sites.