x: add x_discard_events

Also, during session_init, discard old events after we grab the server.
If we don't do that, old, out dated CreateNotify will cause us to add
the window with the same id multiple times. Here is an explanation of
how that could happen:

1. compton connects to X
2. a new window created by someone else, X send us a CreateNotify
3. compton is initializing, so it didn't handle the event.
4. compton pulls a list of all windows from X. This will include that
   window created in (2)
5. compton starts to handle event. it will handle the CreateNotify sent
   in (2), thus adds the same window twice.

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

src/x.h

@@ -110,6 +110,15 @@ static inline winprop_t wid_get_prop(const session_t *ps, xcb_window_t wid, xcb_
 	return wid_get_prop_adv(ps, wid, atom, 0L, length, rtype, rformat);
 }
 
+/// Discard all X events in queue or in flight. Should only be used when the server is
+/// grabbed
+static inline void x_discard_events(xcb_connection_t *c) {
+	xcb_generic_event_t *e;
+	while ((e = xcb_poll_for_event(c))) {
+		free(e);
+	}
+}
+
 /**
  * Get the value of a type-<code>xcb_window_t</code> property of a window.
  *