Add support for per-camera object visibility

[McManning]

This is mostly on the Unity side. I already send Unity the list of objects visible from each viewport camera. I just don't know how to handle this within Unity.

• Layers aren't usable - I don't want to override anyone's layer configurations
• Hide/show objects on a prerender per camera? How bad would the performance be on something like this? I know URP doesn't have the typical OnPreRender() support, can I do this in a generic enough way for all render pipelines?