WiP: Add get_gamma to gamma-control

Created by: agx

This adds get_gamma to get the current gamma lut to wlr_gamma_control. This is needed since some things like gnome-desktop (as used by gnome-settings-daemon) want to read out the gamme lut.

TODOs:

• figure out what to do with gamma-control protocol. It's not marked as unstable but e.g. has no docs and isn't in wlr-protocols or even wayland-protocols either. Is if fit for wlroots 1.0? Idea would be to rename to wlr-gamma-control and mark it as unstable. Old gamma-control could be kept for backward compat but are there any users atm?
• don't always use legacy drm interface

Test Plan

• try examples/gamma
• Use phosh's gamma branch (https://source.puri.sm/Librem5/phosh/merge_requests/58) and see gnome nightlight work / monitor the interataction via dbus-monitor