Adjustable gamma for better accessibility

[fff7d1bc]

Feature request

It would be greatly appreciated if we could get the ability to control gamma in a manner similar to xrandr --gamma 1.5 on X11, effectively providing a 150% gamma setting. Such a feature would be highly beneficial for users with visual impairments who find it difficult to discern details in poorly lit images. In the past, I experimented with the VibrantDeck plugin for Decky Loader, and setting all RGB sliders to 150% produced the expected effect of a 1.5 gamma shift. This makes me suspect that including a similar capability in gamescope would be both feasible and valuable for accessibility. Thanks.