clients/desktop-shell: preserve background/panel in clone mode

clients/desktop-shell: preserve background/panel in clone mode

In shared-CRTC clone mode there are several wl_output globals for one
weston_output. Only one panel and background is needed per
weston_output, so the extra wl_outputs do not get their own panel and
background.

When a head is unplugged, the corresponding wl_output is removed. If
that was the wl_output associated with the background and panel
surfaces, we must transfer the ownership to a remaining wl_output that
was a clone to avoid losing the background and panel completely.

The transfer relies on desktop-shell.so implementation to register
background and panel surfaces with the weston_output, not the
weston_head, so it does not actually matter the wl_output used to bind
the surfaces is going away.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>