X11: fix notifyLoop being called in parallel to main loop end

[dzaima]

Currently, the following can happen:

1. Main thread: runs notifyBool.store(false);
2. Auxiliary thread: runs notifyLoop, notifyBool.exchange(true) returns false; writes character to pipe & sets notifyBool to true
3. Main thread: clears pipe

Now the pipe is clear, but notifyBool is true, meaning no notifyLoop call will do anything, and poll() will never respond.

My comment was technically correct as the events corresponding to notifyLoop calls between the poll & notifyBool clears will be processed, but I failed to consider what happens after that :/

[tonsky]

Thanks!