Implement procedural atmospheric scattering from [Sebastien Hillaire's
2020 paper](https://sebh.github.io/publications/egsr2020.pdf). This
approach should scale well even down to mobile hardware, and is
physically accurate.
## Co-author: @mate-h
He helped massively with getting this over the finish line, ensuring
everything was physically correct, correcting several places where I had
misunderstood or misapplied the paper, and improving the performance in
several places as well. Thanks!
## Credits
@aevyrie: helped find numerous bugs and improve the example to best show
off this feature :)
Built off of @mtsr's original branch, which handled the transmittance
lut (arguably the most important part)
## Showcase:
## For followup
- Integrate with pcwalton's volumetrics code
- refactor/reorganize for better integration with other effects
- have atmosphere transmittance affect directional lights
- add support for generating skybox/environment map
---------
Co-authored-by: Emerson Coskey <56370779+EmersonCoskey@users.noreply.github.com>
Co-authored-by: atlv <email@atlasdostal.com>
Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>
Co-authored-by: Emerson Coskey <coskey@emerlabs.net>
Co-authored-by: Máté Homolya <mate.homolya@gmail.com>
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