To allow for better performance when scaling to larger worlds we can't keep
iterating though all entities when checking visibility.
Instead we now have the PhysicalDomain keep track of all entities that are
either observering something, or being observed.
Since Bullet already contains efficient structures for doing space partioning and
collision detections we'll resuse that for visibility checks.
Each entity now has a sphere of "visibility" with which each perceptive entity does
checks against.
When checking visibility, we now use the Domains more extensively.
The base rules are:
* If an entity is a child of a domain-less entity, visiblity for the
child is the same as for the parent.
* If an entity is a child of a domain-holding entity, visibility is
determined by the Domain.
* Entities cannot observe things above a Domain.
* Entities can observe things below a Domain, if the Domain allows it.
These rules are not yet set in stone. We need some more time figuring out
exactly how we want visibility implemented. We're striving for a balance
between a powerful but not too complex system, with good performance.
