Depth fog in Workbench Engine

The workbench engine allows a few rendering options very useful for modelling, such as cavity, shadow, outline... For modelling big spaces with a small amount of detail (such as in video game level design greyboxing), a very simple depth fog would be very useful to better gauge distances.

Values exposed would be the color and opacity (as alpha), as well as start and end distances. I think it would integrate quite nicely along the existing options.