Add Post Processing Clouds

[shadowmage45]

Yeah, why not.

Certainly not going to be fast to work through, but will be interesting to get it functioning.

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• [ ] Find some method for efficient planet-scale data storage. Can't render clouds for a planet unless there is data...
• [ ] Find some method for usable runtime generation of the cloud data (both movement and permutation). Static clouds look bad, mmkay?
• [ ] Include cloud rendering into the atmosphere scattering model -- use of scattering/phase angles, volumetric shadows from clouds.