We've padded the search area a bit, to handle those entities created on
the fringe. We also now check if the bbox is valid before accessing the
radius.
"entity" allows one to better specify how the new entity should be
created. It's optional; if omitted a default entity of the type in "m_type" is
created.
"interval" allows for specification of the interval between ticks. It's
optional.
When an entity has this property it acts a an spawner entity. At an
interval it checks if there are enough of the specified entities
available, and if not it spawn new ones. This allows for some better
gameplay mechanics.
