Attach UserData to rtcScene and retrieve rtcScene from rtcGeometry

[Raildex]

A QoL change would be to have the ability to add scene-specific data to the rtcScene via UserData.
For example textures which are shared between geometries.

What would be nice to have is also the ability to retrieve the rtcScene directly from rtcGeometry.
With that you can directly access the User Data from the scene from the geometry:
rtcGeometry -> rtcScene -> Scene User Data