Rendering for absorption

[Pietersielie]

At the moment, the rendering shader (a ray-marching algorithm) samples values along the ray and returns either the maximum or the average value, which provides a suitable approximation to emission. To account for absorption in the foreground, a new shader will have to be developed, possibly based on radiative transfer equations.

[paoloserra]

@Pietersielie a while back we discussed a possible solution for emission and absorption. The main idea is captured at #256 .