Glass absorbtion [Feature request]

[JeremyLeaf]

Glass absorbtion [Feature request]

[ricktu288]

Two approaches possible here.

1. Integrate with #69, so that the rays are traced step-wise. This will be quite inefficient if the refractive index and absorption rate are constant (but is the only approach if they are not constant).
2. As @JeremyLeaf suggested, trace the rays as usual, but the distance that each segment of ray goes is considered.

For 2., we need to add a loop in addRay that checks whether the ray is emerging within each glass. And hence each glass must implement isInsideGlass, etc as currently do for GRIN glasses. These change is also required for #37 and the "source-over body-merging" approach of defining "absolute refractive index" (see #51).