Fix OSL reference implementation

[virtualritz]

Either fix the OSL code or change aiStandard to match.
Alternatively: clearly state that one or the other is not meant to be an implementation of this standard.

See also #11.

[iliyang]

Hi @virtualritz. The OSL reference implementation included in this repository is incomplete, partly due to the limited list of closures bundled with OSL. Ideally, all necessary closures to reproduce Standard Surface would be built-in, and we are hoping that those will become part of OSL soon. In the mean time, we can at least provide a complete implementation that assumes the closures are available.