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 0c4445ba57
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>

clients/window.c

@@ -4529,6 +4529,7 @@ display_acquire_window_surface(struct display *display,
 	if (surface->buffer_type != WINDOW_BUFFER_TYPE_EGL_WINDOW)
 		return -1;
 
+	widget_get_cairo_surface(window->main_surface->widget);
 	return surface->toysurface->acquire(surface->toysurface, ctx);
 }