Procedural atmospheric scattering

[ecoskey]

Implement procedural atmospheric scattering from Sebastien Hillaire's 2020 paper. This approach should scale well even down to mobile hardware, and is physically accurate.

For followup

• Integrate with pcwalton's volumetrics code
• refactor/reorganize for better integration with other effects
• solve minor color inaccuracy at sunrise

Showcase

Check the example in examples/3d/atmosphere.rs.

Credits

Built off of @mtsr's original branch, which handled the transmittance lut (arguably the most important part, and I still don't understand the parametrization)

[ecoskey]

Gonna put this up for review, I know the sunrise/set colors aren't perfectly accurate, but I've been unable to fix it. I'd be perfectly fine merging this as an experimental module, considering I still need to merge it and pcwalton's volumetrics stuff into a single module as a followup. I'm just really ready to be done with this pr lmao.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.