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>
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