Precomputed atmospheric scattering as described by Nishita93

Always in my research upon scattering of light rendering techniques, i implement this paper with some modifications, i use a volume texture instead of sampling planes and raycast against a sphere inside pixel shader to determine the lookup parameters.
I decided to compute on CPU for easy stream of calculation and eventual ports to PS3 :)

Actually only rayleigh scattering is displayed but it's a good start.