Cogl (as of 0b2b46ce) now only sets the shell surface as toplevel when
the CoglOnscreen is shown.
Without calling wl_shell_surface_set_toplevel the compositor will not
know what role to give to the compositor and thus the stage will not
appear.
When we look to support multiple roles / foreign surfaces we will need
to revisit this call and ensure we only call it when we are working in
the default case.
https://bugzilla.gnome.org/show_bug.cgi?id=703188
5c93c0a1af