Atmospheric scattering

[bfops]

The sky is all one color. Make it a gradient or something.

[bfops]

Clouds as of 22e3afb

[bfops]

Sun in 9b4c188

[bfops]

Needs a better / more realistic progression of colors across the sky. Something like http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter16.html