window: fix display_acquire_window_surface()

window: fix display_acquire_window_surface()

Make sure that display_acquire_window_surface() creates the Cairo
surface as necessary. Otherwise surface->toysurface can be NULL.

This fixes weston-screensaver fullscreen mode. Demo mode was not
affected as it uses window decorations, and so the Cairo surface is
created. This regression was introduced by:

commit 0c4445ba5756e51007606028bce00b2ecb352bd7
Author: Pekka Paalanen <ppaalanen@gmail.com>
Date:   Wed Feb 13 16:17:23 2013 +0200

    window: create Cairo surfaces on demand for redraw

Reported-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>