GLX: Remove the XESetCloseDisplay callback during teardown.

When libGLX.so is unloaded, call XESetCloseDisplay to remove the display close
callback for any displays that still exist.

Otherwise, if libGLX.so is loaded using dlopen, and then unloaded while a
display is still open, then XCloseDisplay would try to call the (now dangling)
callback.
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