Global cloud rendering

@immiao @byumjin this would be a great real-time rendering project for the winter/spring!

# Phase 1 - billboards

* [ ] Procedural billboard texture generation from various cloud types
* [ ] Navie rendering with Cesium's billboard engine
* [ ] Post processing
* [ ] LOD rendering if needed

# Phase 2 - Volumetric clouds

* [ ] Procedural generation from noise, etc.
* [ ] GPU ray marched rendering with secondary ray effects

# Later

* [ ] Connect to real-time weather data

# Resources

**Inspiration from GeoFS**

* https://twitter.com/GeoFlightSim/status/913410574146572289
* https://twitter.com/GeoFlightSim/status/918851600348844032
* https://twitter.com/GeoFlightSim/status/916324430476972033

@ron-popov and @yotam180 may have input.

**Cloud rendering**

* [Volumetric Clouds](http://bitsquid.blogspot.com/2016/07/volumetric-clouds.html). 2016
* [Real-Time Cloud Rendering](http://www.markmark.net/PDFs/RTClouds_HarrisEG2001.pdf). Eurographics 2001. Early and influential work using billboards
* [Nubis: Authoring Real-Time Volumetric Cloudscapes with the Decima Engine](http://www.advances.realtimerendering.com/s2017/index.html). SIGGRAPH 2017
* [The Real-Time Volumetric Cloudscapes of Horizon Zero Dawn](https://www.guerrilla-games.com/read/the-real-time-volumetric-cloudscapes-of-horizon-zero-dawn). SIGGRAPH 2015. @MegSesh may have feedback on this approach
* [Modeling, Lighting, and Rendering Techniques for Volumetric Clouds](http://www.gameenginegems.net/gemsdb/article.php?id=30). Game Engine Gems 2
* Lots of papers and apps at [vterrain.org](http://vterrain.org/Atmosphere/Clouds/)
* Resources from the [twitterverse](https://twitter.com/pjcozzi/status/926105146534367232)
* [Dissecting “Tiny Clouds”](https://blog.demofox.org/2017/11/26/dissecting-tiny-clouds/)

**Cesium rendering engine**

* [Rendering a Frame](http://cesiumjs.org/2015/05/14/Graphics-Tech-in-Cesium/)
* [Renderer Design](http://cesiumjs.org/2015/05/15/Graphics-Tech-in-Cesium-Architecture/)
* [The Graphics Stack](http://cesiumjs.org/2015/05/26/Graphics-Tech-in-Cesium-Stack/)

No source file, test, or concrete entry point is named. Start by reading the listed Cesium rendering-engine resources and inspecting the billboard engine before evaluating the Phase 1 requirements. Done is not narrowly defined: the proposal spans procedural textures, billboard rendering, post-processing, optional LOD, volumetric ray marching, and weather-data integration.

Written by the indexing model from the issue text.