compositor-wayland: fix mode_list corruption on --sprawl

compositor-wayland: fix mode_list corruption on --sprawl

The wayland-backend with --sprawl is one way to trigger
wayland_output_create_for_parent_output(), which intends to find a mode
from the parent mode list and use it. Calling wayland_output_set_size()
initialized an embedded struct weston_mode and inserts that into the
mode list. Then the assignment output->mode = *mode; corrupts the
mode_list by overwriting the link entry. This leads to an endless loop
in bind_output() in compositor.c.

Fix this by manually doing the setup that wayland_output_set_size() did
and do not call it.

As a side effect, it now relays the parent compositor's physical output
size to our own clients. It no longer smashes the refresh rate either.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Daniel Stone <daniels@collabora.com>