gamma-control fails when an output is disabled

[emersion]

1. Run a wlr-gamma-control client, like redshift
2. Turn an output off
3. The client sets a gamma
4. wlroots makes it fail because it's not possible to set gamma for a disabled output

The client doesn't know that the output is disabled, so it can't wait for the output to be re-enabled. Instead, gamma-control should probably stash the pending gamma LUT somewhere, and apply it next time the output is enabled.

[emersion]

Note, we can't make the set_gamma call succeed when the output is disabled, because the output won't have a CRTC associated.

[emersion]

Sway users: swaywm/sway#5791