Cloud sphere

Preview branch for local commenting and tracking

There's still 
- [ ] lighting (day/night), 
- [ ] shading (some kind of BDRF function?), 
- [ ] fading (don't draw it when you're close/in it), 
- [ ] layers (draw multiple layers of clouds)... 
- [ ] and of course generating it procedurally from the planet data itself 
- [ ] and then updating it in realtime... which might be tricky.