Add scene side discrete level of detail.
New node `LOD` for UI, and `LODManager` within `World` for automatically updating child visibilities based on distance from cameras.
`const` is used on all methods, even when they cause modification of the server. This reworks the methods of the server to only use `const` on method that don't change the state of the server.
- DarkKilauea
a0715b30f9