desktop-shell: handle redundant panels

desktop-shell: handle redundant panels

If for some reason the helper client weston-desktop-shell would create
more than one panel surface for the same weston_output, this code would
corrupt the surface destroy listener list by adding a link already in
one list into another list.

Instead, do not store the new, redundant panel surface and do not
subscribe to its destruction. Also, tell the helper that the surface is
redundant by configuring it with a 0x0 size, so that we don't waste
memory on a panel that is never used.

(Clone mode is a valid reason why weston-desktop-shell could do that.)

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