Factory for creating new tracker instances. As new items are detected, each distinct item
(as indicated by an item ID generated by the detector) will have a new instance of the
tracker created for that item. A tracker will receive all notifications for that item for
consecutive frames, until the item has been determined to be no longer present.
is called on a tracker instance, no future notifications will be sent to that instance.
Associated resources, if any, should be freed upon receiving that call.