VDF Support #310

MaterialX expects mixable VDFs.

OpenPBR also expects volume shading to be unified with surface shading.

In MoonRay, volume shaders (aside from subsurface, as a special case) are totally distinct from materials and surface shaders, and assigned directly to geometry through RDL layers. A shader class also cannot be both a Material and a VolumeMaterial, thanks to diamond inheritance. This will likely need to be redesigned, with changes spanning from Scene_rdl2 through all MoonRay repos.