Add Light Volumes scene:

- Add scene to support Light Volumes: https://github.com/REDSIM/VRCLightVolumes
  - Scene is baked differently from main scene, as it uses Progressive GPU instead of CPU.
  - Scene has inactive lightboxes with Light Volumes disabled.
- This scene is selected by default if the Light Volumes package version 0.7.2 or higher is installed.

Author: hai-vr