[feature] Expose setting other IoR

[Derpius]

Describe the solution you'd like
Method of setting the IoR outside of the hit surface.

At the moment every interaction is air-dielectric or air-conductor, and while that's fine for conductors (and the diffuse retroreflection) it's not suitable for specular transmissive objects in water for instance, or other nested dielectrics.

Most likely this would be a function on the BSDFMaterial as it was before.