Entities that are expensive to generate, like skeletal models, can be deferred until their bounds are found to be in view, in the frustum of a shadowing light that is in view, or contacted by a trace / overlay test. This is also used to do visual cueing on items in the view The renderView may be NULL if the callback is being issued for a non-view related source. The callback function should clear renderEntity->callback if it doesn't want to be called again next time the entity is referenced (ie, if the callback has now made the entity valid until the next updateEntity)