Philipp Zabel authored Mar 25, 2022 and Derek Foreman committed Sep 28, 2023

Insert the backend into the weston_compositor::backend_list instead
of setting weston_compositor::backend. The compositor uses this to
determine whether the backend is capable of being loaded simultaneously
with other backends.

The headless backend can only be loaded as primary backend.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>