This AABB tree required when need to filter out input mesh that has not
influence navmesh tile output. This filtering is already done before. Each
recast mesh corresponds to a single navmesh tile and has appropriate bounds.
Use LRU modification to hold currently used items. Use RecastMesh binary
data for item key.
Store original pointer of btCollisionShape in user pointer to make available
it as an identifier within all duplicates. Use pointer to heights data array
for btHeightfieldTerrainShape.
