It is intended to replace using GetResources() and ChangeBacklight().
It moves from a normalized 1-100 numbers, to directly exposing the
hardware. This more closely maps to how gsd-backlight.c in
gnome-settings-daemon normally works, and simplifies the API a bit to
not have to deal with rounding issues.
There is still no KMS uAPI for this, so it still only hooks up to
XRANDR. Being private API, it doesn't try very hard to predict how the
KMS uAPI will look. When that day comes, it will likely need some
adaptations.
Part of the motivation here is to get something for gsd-backlight.c to
use where it can work more similarly to how the current common case
(sysefs) works, while attempting to migrate away from libgnome-rr.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3861>
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