The modesetting driver is missing gamma support
Steps to reproduce:
1. With modesetting driver installed, Launch any SDL game such as OpenArena or Urban Terror
2. Adjust gamma settings
Game world is visible
Gamma levels do not change leaving all games too dark to play
In my testing, Nvidia proprietary, xf86-video-intel, xf86-video-ati, and xf86-video-nouveau all support gamma controls. Only modesetting seems to lack this essental feature for playing SDL based games.
Fixed with https://cgit.freedesktop.org/xorg/xserver/commit/?id=ac138f9b31b0fba00742edbc3326afe66e28099a