nested: Disable cairo on the subsurfaces

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.