Return extent from ol.extent.extend
Add ol.extent.getArea
Add ol.extent.getEnlargedArea
Add ol.extent.getIntersectionArea
Add ol.extent.getMargin
Add ol.structs.RBush
Port ol.source.Vector to ol.structs.RBush
Port rtree example to ol.structs.RBush
Move ol.structs.RTree out of the way
Add ol.structs.RBush#assertValid
Refactor ol.structs.RBush tests
Don't check for under-full nodes when validating ol.structs.RBush

Removal can lead to under-full nodes.
Implement ol.structs.RBush#remove_
Add FIXME
Add ol.structs.RBush#update
Keep R-Tree up-to-date in ol.source.Vector
Add optional destination argument to ol.extent.clone
Clone extents in ol.structs.RBush to prevent modification