Before loading a backend, clear the weston_compositor::backend pointer
to check whether the backend supports multi-backend operation and adds
itself to the weston_compositor::backend_list.
Keep weston_compositor::backend pointing to the last loaded backend
either way, to allow the calling compositor code to store it away for
later, to check whether a head belongs to a given backend in the output
configuration code. This workaround can be removed after all backends
are converted to be multi-backend aware.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
20c5b4527c