Called by a snip to notify the administrator that the specified snip has changed its display size (without being polled by get-extent). The snip generally needs to be updated after a resize, but the snip decides whether the update should occur immediately.
The method call is ignored if the given snip is not managed by this administrator.
If refresh? is not #f, then the snip is requesting to be updated immediately. Otherwise, needs-updatemust eventually be called as well.