Some events, such as the display.delete_id, aren't very urgent and we
would like to not always send them immdiately and cause an unnecessary
context switch. The wl_resource_queue_event() function will place the
event in the connection output buffer but not request the main loop to
poll for writable. The effect is that the event will just sit in the
output buffer until a more important event comes around and requires
flushing.
We need to make sure the client doesn't reuse an object ID until the
server has seen the destroy request. When a client destroys an ID
the server will now respond with the display.delete_id event, which lets
the client block reuse until it receives the event.
So obvious in retrospect. The object system can do all the work for us
and keep track of pending calls as regular objects and we don't need to
abuse the resource system to get them cleaned up on client exit. We
don't need the custom key management or (broken) lookup, we just sue
object IDs. And last but not least, anybody can receive the callback,
not just display listeners.
