_generateGuid = function ( tracker, gPoint ) {
            return tracker.hash.toString() + gPoint.type + gPoint.id.toString();
        }
Provides continuous computation of velocity (speed and direction) of active pointers.
This is a singleton, used by all MouseTracker instances, as it is unlikely there will ever be more than
two active gesture pointers at a time.

@private
@member gesturePointVelocityTracker
@memberof OpenSeadragon.MouseTracker