More Gas

Added turbulence for wispiness.  Also implemented the Henyey-Greenstein scattering function, which seem to provide more realistic scattering. The effect of the blue beam on the gas is much more pronounced now, since the scattering is no longer isotropic.

Here's an example of the beam cutting through a thick volume:

Now, inside an infinite volume:

Unfortunately, the previous image seems over-saturated, possibly a result of bad MLT luminance estimation.  Finally, here's a pretty good one:

The clouds are starting to look really great.  Unfortunately, it looks like you can't even see the asteroids in that last shot (there are actually 100 of them nearby...I guess the gas is just really thick).  Scene composition with this thick gas will be difficult, but it'll be a huge payoff if I can get it right.  That gas looks far better than what I ever imagined I would be able to achieve!