* Anisotropic BSDF now supports GGX and Beckmann distributions, Ward has been
removed because other distributions are superior.
* GGX is now the default distribution for all glossy and anisotropic nodes,
since it looks good, has low noise and is fast to evaluate.
* Ashikhmin-Shirley is now available in the Glossy BSDF.
* Simplify default color values, where each component was the same.
* Initialize closures as Null Closure, rather than assigning an existing closure, gets overwritten anyways.
* Moved kernel/osl/nodes to kernel/shaders
* Renamed standard attributes to use geom:, particle:, object: prefixes
* Update stdosl.h to properly reflect the closures we support
* Fix the wrong stdosl.h being used for building shaders
* Add geom:numpolyvertices, geom:trianglevertices, geom:polyvertices attributes
