Gaussian Splat Re-lighting and Proxy-based Shading Dynamics

Providing a clean and convenient API for the relighting of splats as well as for using proxy meshes to drive shading dynamics would greatly enhance the experience.

I came to this realization when attempting to port this PC project which uses a proxy mesh.

Once these APIs are added, canonical examples in the docs would be of great value to the community.

cc @Cedric