libweston: Avoid using the surface's output when emitting a timeline

This is an odd corner case where the surface doesn't yet have an output
assigned -- noticed when starting up with the RDP backend and
fullscreen-shell, and we attempt to emit a timeline point for a surface
without an output assigned, causing weston to crash when that
happens.

Rather than trying to catch this in the timeline code, still emit the
flush damage timeline but instead of using the output the surface is on,
use the output that accumulates damage.

Suggested-by: Derek Foreman <derek.foreman@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>