If the current thread isn't the foreground thread. Otherwise we may
clip the cursor in the wrong thread, that isn't expecting mouse messages
or may not be checking its messages.

Red Faction has some race condition where this can happen for instance,
with clip_fullscreen_window called from X11DRV_DisplayDevices_Update
callback in a background thread. This thread starts clipping the cursor,
and the foreground thread isn't receiving MotionNotify events anymore.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>