This is intended to be used only for plane assignment, and CRTC like
changes, so that one can e.g. change a cursor plane on a pending update
that changes the primary plane, before it has been committed to KMS.
The kms-updates test overrides the get-state function MetaKmsCrtc. This
is needd to not have the update mechanism not clamp the gamma size to 0,
as vkms reports the gamma length 0. By pretending it's 3, we can test a
simple and small gamma lut is merged correctly when merging updates.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2854>
The way device backends implement power saving differ, and power saving
needs to contain nothing incompatible in the same update. Make it
impossible to e.g. mode set, page flip, etc while entering power save by
not using MetaKmsUpdate's at all for this.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2159>
