Change screen brightness in perceptual space
These changes make the screen brightness setting to operate in perceptual space (CIE L*).
The difference can be seen at https://share.nereid.pl/i/exi914vj.jpg The image shows a series of photos of a laptop screen, performed with the same camera exposure settings. The top images are at 0% brightness, while the bottom ones are at 100%. The intermediate images were done with 5% brightness increments. The left column shows the current KDE behavior. The right column shows how things look after changes from this MR are applied. You can easily observe that the right column shows a smooth change in screen brightness.
Note that the photos were taken in the proof-of-concept phase, and a slightly different power curve (1/2.2) was used. The differences are not significant.
Please see the source code and commit messages for implementation details.