A racing reader could read from our fd between SDL_IOReady()/X11_Pending()
and our call to XNextEvent() which will cause XNextEvent() to block for
more data. Avoid this by using XCheckIfEvent() which will never block.
This also fixes a bug where we could poll() for data, even when events were
already read and pending in the queue. Unlike the Wayland implementation,
this isn't totally thread-safe because nothing prevents a racing reader
from reading events into the queue between our XCheckIfEvent() and
SDL_IOReady() calls, but I think this is the best we can do with Xlib.
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