[Atmosphere] Implement an atmosphere simulation

[Razakhel]

Many resources are available, which need to be tested further:

• Scratchapixel's explanation: https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky
• Frostbite's papier on physically-based sky, atmosphere & clouds: https://sebh.github.io/publications/egsr2020.pdf
  • Related presentation: https://www.ea.com/frostbite/news/physically-based-sky-atmosphere-and-cloud-rendering
• GPU Gem about atmospheric scattering: https://developer.nvidia.com/gpugems/gpugems2/part-ii-shading-lighting-and-shadows/chapter-16-accurate-atmospheric-scattering

[Razakhel]

Implementation attempts using RaZ are available on an external dedicated repo: https://github.com/Razakhel/Atmos