Don't hold the xlib-xcb lock while sleeping: that allows deadlock.

With this patch, `ico -threads 2` runs without deadlock.

Many thanks to Christoph Pfister <christophpfister@gmail.com> for
pointing out the problem, providing detailed analyses, explaining it to
me repeatedly until I understood what was going on, and proposing and
reviewing possible solutions.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Acked-by: Christoph Pfister <christophpfister@gmail.com>