xdg-output: destroy outputs before manager
xdg-output: destroy outputs before manager

Since output_destroy() calls wl_list_remove() on the output's link,
the manager must still be valid. This is the same bug fixed in bf926e3
but with a different interface.

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots