Where the dragged waypoint is now, as [longitude, latitude].
'dragstart' once, 'drag' on every move, then 'dragend' where the waypoint is dropped.
The dragged waypoint's index in the waypoints, null placeholders included. For an
'insert', the index the new waypoint was inserted at.
One step of a drag, as RouteDraggingOptions.onWaypointsChange reports it.