[Enhancement Request]: Standard Surface implementation

Hey,

Thanks a lot for opening up the code 😄 I'm very grateful as it a very useful reference on how one can implement the Autodesk Standard Surface model. Such an implement of a multi-lobed BSDF model is hard to find online.

Looking through the code, it appears Aurora's implementation of the Standard Surface model diverges from the [specification](https://autodesk.github.io/standard-surface/) as well as Arnold in some key places like:

1. Delta distribution is used for transmission rather than GGX. As a result, rough transmission for glass isn't supported.
2. Volumetric properties aren't handled yet. It would be useful for absorption/scattering as well as brute force SSS.
3. Subsurface uses diffuse transmission even if the thin walled property is false (The spec doesn't actually specify a model to be used though)

The readme says "..new features and enhancements to performance and quality planned" so my questions would be:

1. Are there any plans to bring the material model closer to how its implemented in Arnold?
2. Is there a list of planned features/enhancements that can be openly shared with the community?

Thank you.

The issue does not identify a file, test, or entry point. Begin by locating Aurora's Standard Surface implementation, then compare its transmission, volumetric, and subsurface behavior with the linked specification and Arnold; completion would require an agreed scope and corresponding validation.

Written by the indexing model from the issue text.