Derek Foreman authored Jun 13, 2023 and Pekka Paalanen committed Jun 19, 2023

Until now we've only had the unadorned arithmetic functions, but they're
easy to abuse and tedious to use.

For now, we just add weston_coord_global_add/sub functions and use them
where appropriate.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>