If a window already has a buffer attached by the time we figure out it
exists and sets its role, we missed the initial commit and will never
get it. For windows that animate interactively, that's fine, as we'll
get another one soon enough, but 2D apps like launchers repaint on
demand, so it's not fine there.
frames out of thin air.
Also use dummy ClientMessage events to nudge steamcompmgr's event loop,
Expose events have a meaning and we don't want to confuse some clients.
