Inconsistent use of `VolumeMaterialInteraction` thickness

`VolumeMaterialInteraction` seems to be used in two cases: material recording and dense environment energy loss.

In the material recording case we expect the thickness to be set to `1` because we later rescale it to the step length.

But the dense environment expects a properly scaled material slab which is directly used for the energy loss.

We should decide for one of the solutions. IMO the prescaled one is the less confusing one