Smooth ramped backlight transitions

When swapping between macOS and Linux on a MacBook Pro it is noticeable that on Linux the backlight moves in steps and on macOS it ramps between values.

Ramping between values makes for a nicer experience, in my opinion in all cases. I would argue that it is a nicety when users change the backlight level, but that it is most distracting when the light sensor triggers it. I have no idea whether that is g-s-d or something else that does this, but I assume g-s-d?

Maybe relevant to !14 (merged)?