Per-instance materials

I have faced a problem in OSPRay and just would like to make sure it is not an issue for ANARI. Let’s assume we want to create a scene with two objects that share geometry but have different position and material (e.g. wooden teapot and gold teapot). We want to use a single geometry and thus construct a single BVH. This means we will have a single `anari::Group` object. In the current 1.0 ANARI version there is no way we can specify per-instance material. Will it be solved by defining another instance sub-type e.g. `KHR_INSTANCE_TRANSFORM_AND_MATERIAL` which would additionally contain index into some material array? But then, where to store this material array, in `anari::Surface`?

Review the ANARI 1.0 instance, anari::Group, and anari::Surface concepts described in the issue, then compare them with the existing ANARI-Docs API documentation. Clarify whether per-instance materials are supported and where material references or arrays should belong; done means the API behavior and any required subtype are documented with an agreed design.

Written by the indexing model from the issue text.