Two-sided Surface extension

[progschj]

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.