#5 Implement atmospheric rendering

[Gordon Tisher]

Implement atmospheric rendering, probably using the approach in http://www.vis.uni-stuttgart.de/~schafhts/HomePage/pubs/wscg07-schafhitzel.pdf

The spherical quadtree code can probably be used for this, with the winding reversed.

This allows two cases:

- Outside the atmosphere, we don't see the atmosphere in front of the planet. We only see the inside of the thin ring of atmosphere around the edge of the planet (the quadtree normal check allows for a bit of wrapping around). The color of the inside faces we see is determined by the chord between them and the viewpoint.

The planetary surface shader (Periapsis/data/Shaders/lithosphere_vertex_shader.glsl) should attenuate the planetary surface to look like there's atmosphere between it and the viewpoint.

- Inside the atmosphere, we attenuate the planetary surface, as well as the inside of the sphere above us.