This commit introduces wlr_output_schedule_done and refactors the mechanism
used to send wl_output events to clients.

wlr_output_schedule_done schedules a wl_output.done event. This allows clients
to see wlr_output property changes as atomic.

This function is also useful for add-on interfaces like xdg_output which need
to trigger a wl_output.done event to apply their new state.