Volumetric lighting

[vlj]

This PR adds volumetric lighting for sunlight and point lights.

• The volumetric effect is raymarch based and is based on physical transmittance equation
• The fog is "thicker" around z = 0, and thickness follow an exponential curve
• sunlight has shadow support (but it seems quite costly)

This is a draft PR.

TODO:

• Currently only Vulkan is supported
• Add options to disable or limit the number of raymarch steps

Screenshots

[MaNGusT-]

seems that it's time to change fog color :). awesome work btw

[MaNGusT-]

got huge fps drop when returned from 1st beta away mission. Log was about 1.5gb with these errors
info |10:11:47: [operator():56] Assert in Warzone: D:/a/warzone2100/warzone2100/src/lib/ivis_opengl/pielighting.cpp:56 (y < mapHeight + 1), last script event: ''

info |10:11:47: [operator():55] mapTile: x value is too big (120,67) in 64x64

info |10:11:47: [operator():55] Assert in Warzone: D:/a/warzone2100/warzone2100/src/lib/ivis_opengl/pielighting.cpp:55 (x < mapWidth + 1), last script event: ''

info |10:11:47: [operator():56] mapTile: y value is too big (120,67) in 64x64

[MaNGusT-]

It's better to use gaussian specular term in tcmask shader because it does sharper light reflection and looks better on metallic surfaces
https://paroj.github.io/gltut/Illumination/Tut11%20Gaussian.html