WorldPartition-like functionality

[JMS55]

Unreal has a WorldPartition feature, where you can divide the static portions of your game level into grid cells. Cells not near a "streaming source" (e.g. the player) get unloaded from memory. Cells that become newly visible to a streaming source load in from disk.

I think it would be great to make a new crate extending big_space, and build upon the existing Grid and GridCell functionality to support streaming cells.

Theoretical implementation plan:

• Support instantiating a GridCell from a BSN file
• Add a GridCellStreamingSource component
• Add a "streaming" flag to Grid
  • When enabled, cell entities not visible to a streaming source should be despawned
  • When enabled, cell entities newly visible to a streaming source should be spawned as a child of a BigSpace
• Create some kind of tool for baking foo.bsn files representing a GridCell into a separate foo.imposter.bsn file
  • Create a few simplified meshes based on the static landscape kind of entities present in the original file
  • Store the new LODs as a VisibilityRange in the imposter file
  • When a Grid has streaming enabled, and a cell is despawned, spawn in the imposter cell
  • When a Grid has streaming enabled, and a cell is newly visible, despawn the imposter cell and spawn in the real cell