Two-sided Surface extension

Inspired by this: https://github.com/NVIDIA/VisRTX/issues/63

To accomodate face culling an extension could add additional material parameters `materialFront`, `materialBack` for surface objects. They would override the stock `material` parameter for that side of the surface.

Back/front face culling would then be achieved by setting a fully transparent material on that side. Additionally the extension could specify that setting no material is valid and define the behavior to transparency.

Start by reading the linked VisRTX issue and the proposal here for the intended two-sided surface behavior. Define the extension's front and back material parameters, face-culling approach, and behavior when no material is set; completion requires an agreed specification for these cases.

Written by the indexing model from the issue text.