Add a setting to pin the vertex fog contribution (and adjust it for any given map/scene)

[rdw-software]

As mentioned in #373, Doddler has introduced an interesting approach to pinning the vertex fog contribution:

• instead of computing the contribution based on the distance between camera and vertex position, pin it to some "average"
• This makes the look and feel of all scenes more consistent as zooming in or out doesn't change the color grading
• Unfortunately it may produce visibly inaccurate results and make debugging rendering issues more difficult, as well
• It's still an interesting idea and so an option to use this approach (and configure the exact contribution) would be neat

Probably requires some UI to make it less fiddly to use, which I haven't looked into yet beyond the initial RML UI prototype.