vops pathtracer bsdfs

Added some different bsdfs to my houdini vopsop pathtracer, trying to see how far I can push things. Now rather than just lambertian surfaces, it can do perfect specular (mirror) reflection using the builtin reflect vop, as well as a fresnel dielectric bxdf that does reflection/refraction, importance sampled with the fresnel term. For some reason houdini's built in fresnel vop isn't accessible for me in the geometry (sops) context, so I used a vopnet to reconstruct schlick's approximation.

Of course being a pathtracer, caustics come without any extra effort either (though it's a fair bit slower to converge than purely diffuse surfaces). The animations here aren't real-time - the geometry has been cached in advance using the cache sop.