ClutterX11TexturePixmap calls get_allocation_box() when queueing a
clipped redraw. If the allocation is not valid, and if we queue a
lot of redraws in response to a series of damage events, the net
result is that we spend all our time in a re-layout. We can
short-circuit this by checking if the actor has a valid allocation, and
if not, just queue a redraw - the actor will be allocated by the time it
is going to be painted.
Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>