// requires: non-null callback method or listener class
// effects: adds the callback to the list of composite cbs
public void AddCompositeChangedCallback(CompositeChangedCallback callback)
{
_compositeChangeCbs.Add(callback);
}
// requires: non-null callback method or listener class
// effects: removes the callback from the list of composite image cbs; noop if not found
public void RemoveCompositeChangedCallback(CompositeChangedCallback callback)
{
_compositeChangeCbs.Remove(callback);
}
