Add gamma adjustment functionality to the RGB curves.

[LinearCycle]

Hello. I greatly appreciate Darktable for its convenient curve adjustments in linear gamma. However, when making brightness-related adjustments, I've noticed that the cubic spline interpolation curve doesn't quite meet my needs—it lacks sufficient “smoothness” and physical accuracy. The gamma presets also lack flexibility. I would like to see a gamma adjustment added that allows modifying the overall curve gamma in linear space via a slider or curve.

1. Add a slider below the curve, similar to NX Studio. (As shown in the image: gamma slider)
2. Include a power law function option under “Interpolation Method,” limiting the number of points to 3.

I would be very grateful if this could be implemented. Thank you for your efforts. Have a great day.