Add skin tone line to vectorscope

[michmill1970]

Add a skin-tone line to the vectorscope in the darkroom view.

All skintones fall along a straight line in a vectorscope view. The paler the complexion, the closer to the middle of the scope, and darker complexions will have an elongated distribution, but all complexions will fall along a very similar line. Some may be slightly above the line, and some complexions may fall a very small amount below.

This line will help considerably when color grading portraits, especially when dealing with odd lighting.

I use this in Davinci Resolve, and it's an incredibly useful feature.

[ralfbrown]

Previously discussed in #11467, along with alternatives. There is in fact not a common tone among all ethnicities.

[MStraeten]

btw: darktable 4.4 added color harmony indicators to the vectorscope - so it’s quite easy to show something like a skin tone value line using a monochromatic indicator, rotating it to a proper value and reducing the width according to user’s preferences