nested: Disable cairo on the subsurfaces

The subsurface widgets on the nested example aren't using Cairo to
render so we should turn it off to prevent the toy toolkit from
creating a redundant extra surface for it. This is particularly
important since Mesa commit 6c9d6898fdfd7e2 because the surface that
the toolkit tries to create is zero-sized and that patch prevents that
from working. This was causing weston-nested to crash.

clients/nested.c

@@ -976,6 +976,8 @@ ss_surface_init(struct nested_surface *surface)
 				      nested,
 				      SUBSURFACE_SYNCHRONIZED);
 
+	widget_set_use_cairo(ss_surface->widget, 0);
+
 	ss_surface->surface = widget_get_wl_surface(ss_surface->widget);
 	ss_surface->subsurface = widget_get_wl_subsurface(ss_surface->widget);