tickit_window_expose() marks the given area of the given window as needing to be re-rendered, causing it
to receive a TICKIT_EV_EXPOSE event when tickit_window_flush(3) is next called. exposed may be NULL,
indicating that the entire window should be exposed.
If the window, or any of its parents, are hidden, then this function has no effect. Otherwise, it
enqueues the corresponding area on the root window as being damaged, causing an TICKIT_EV_EXPOSE event to
propagate upwards from the root the next time tickit_window_flush(3) is called. This will propagate up to
any window occupying that area, meaning that this window or others may receive it.