libweston: move weston_output::mode_list init to core

Initialize the list in weston_output_init() instead of doing it
separately in each backend.

One would expect weston_output_init() to initialize all weston_output
members, at least those that are not NULL.

We rely on the set_size() functions to be called only once, as is
assert()'d. If set_size() becomes callable multiple times, this patch
will force them to be fixed to properly manage the mode list instead of
losing all members.

compositor-wayland.c is strange in
wayland_output_create_for_parent_output(): it first called
wayland_output_set_size() that initialized the mode list with a single
mode manufactured from width and height and set that mode as current.
Then it continued to reset the mode list and adding the list of modes
from the parent output, leaving the current mode left to point to a mode
struct that is no longer in the mode list and with a broken 'link'
element. This patch changes things such that the manufactured mode is
left in the list, and the parent mode list is added. This is probably
not quite right either.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>