colors/colordevice: make channel factors linear

The redshift table is in gamma 2.2 encoding and not linear, which means that it only yields correct results with 1.0 pixel values. It also means that when it's being applied in linear space in the color management shaders, the result is quite wrong.

To fix that, this commit makes the channel factors linear and the backend calculates the nonlinear factors where needed.

(cherry picked from commit 36bec2d9)