core: X11's update sometimes fails while polling for events

[RonaldZielaznicki]

[xcb] Unknown sequence number while processing queue
[xcb] You called XInitThreads, this is not your fault
[xcb] Aborting, sorry about that.
core-triangle: ../../src/xcb_io.c:278: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
run-triangle
└─ run core-triangle failure

It's pretty random when this happens, but I did isolate it to the Update method. If I don't touch the keyboard, I can't reproduce the error.

But, if I scroll the mouse, move windows; I can eventually get the above error to occur.

This error will also occur during deinit. But, is hidden by a double free error that is fixed in #1289.