This commit makes more output properties (mode, enabled, scale and transform)
atomic. This means that they are double-buffered and only applied on commit.
Compositors now need to call wlr_output_commit after setting any of those
properties.
Internally, backends still apply properties sequentially. The behaviour should
be exactly the same as before. Future commits will update some backends to take
advantage of the atomic interface. Some backends are non-atomic by design, e.g.
the X11 backend or the legacy DRM backend.
Updates: https://github.com/swaywm/wlroots/issues/1640
ee5f98ad49