Perform redisplay.
Optional arg FORCE, if non-nil, prevents redisplay from being
preempted by arriving input, even if `redisplay-dont-pause' is nil.
If `redisplay-dont-pause' is non-nil (the default), redisplay is never
preempted by arriving input, so FORCE does nothing.
Return t if redisplay was performed, nil if redisplay was preempted
immediately by pending input.
(fn &optional FORCE)