Add WorldRenderer parameter to WorldLoaded.
Introduce ScreenMap trait for caching screen-coord queries.
Pull ActorMap back out into a trait.
Make ActorMap addition explicit.
Track actor positions in ActorMap.
Remove SpatialBins.
Replace dynamic Actor.Bounds with the (unchanging) relative rect.
Merge pull request #3861 from pchote/spatialbins-cleanup

Overhaul spatial caching