Atmosphere PBR support and volumetric shadows tracking issue

[mate-h]

What problem does this solve or what need does it fill?

The current version of the atmosphere pipeline does not support space views or affect PBR directional lights. I want to have a realistic physically based lighting model instead of just a composited post processing effect.

What solution would you like?

• Switch to using a spherical coordinate system should be supported with a custom origin point
• Add ray-marched rendering to the atmosphere to support space-views
• Directional lights should be affected by sampling the transmittance texture, for instance objects lit with directional lights should turn orange at sunset.
• Add environment map reflections dynamically generated by the atmosphere renderer, plug it into the environment filtering pipeline to get the diffuse environment map lighting information and specular reflections mip chain at different roughness levels.
• Expose control to the user for the angular size of the sun.

Additional context

We landed the environment map filtering in this pull request: #19076

Therefore the Atmosphere PBR integration PR became unblocked: #19037

We saw overall positive feedback for this pull request (13 ❤️ ! ) and it makes me think that it's an important feature to land in Bevy.

I continued work in another PR in my branch of Bevy and got some useful feedback: mate-h#10

The overall longer term goal is outlined in this issue: #18151

Conclusion

This is a tracking issue in the Bevy repo and will try a different approach this time to merge this. I will be posting a series of smaller PRs that reduces the burden on maintainers to review large changes.

[mate-h]

Current open issues for atmosphere:

• Atmosphere does not render below a certain height #19131
• Transparent meshes with nothing opaque behind it don't render parts of it when below sea level with Atmosphere on #19159
• New atmosphere crashes if hdr: true is not specified #18959

Current open PRs:

• Use RenderStartup for atmosphere rendering. #20186
• 🧪 Create bevy_volumetrics #18708

[mate-h]

Additional tasks noted by @JMS55 :

• generating motion vectors for the atmosphere for TAA and DLSS: can be the same as camera motion vectors. could get tricky for space views.
• add atmospheric/volumetric shaders rendering support in Solari 🤩 : we need to add the atmosphere shaders in the pathtracer, this is super useful for producing beautiful volumetric shadows and obtaining ground truth images to compare against for visual testing.