libweston: extend output->region lifetime

It's a little awkward to try to keep the weston_output::region and
weston_output::previous_damage allocate exactly only when the output is
enabled. There was also a leak: weston_output_move() was calling
weston_output_init_geometry() on an already allocated regions without
fini in between.

Fix both issues by allocating the regions in weston_output_init(),
always fini/init'ing them in weston_output_init_geometry(), and fini'ing
for good in weston_output_destroy().

This nicely gets rid of weston_output_enable_undo() so I do not need to
try to figure out what to do with it later.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>