core: unmap overlay window when we first acquire it

It used to be unmap when we receive its MapNotify, but now since we discard
events received before we grab X server, that event it lost. But it
turns out we can just unmap it when it's first created, no need to wait
for the MapNotify.

Partially fix #160

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -169,7 +169,6 @@ static inline void ev_focus_out(session_t *ps, xcb_focus_out_event_t *ev) {
 
 static inline void ev_create_notify(session_t *ps, xcb_create_notify_event_t *ev) {
 	assert(ev->parent == ps->root);
-	// TODO delay fill_win
 	add_win_top(ps, ev->window);
 }