function updatePointerCaptured( tracker, gPoint, isCaptured ) {
        var pointsList = tracker.getActivePointersListByType( gPoint.type );
        var updateGPoint = pointsList.getById( gPoint.id );

        if ( updateGPoint ) {
            if ( isCaptured && !updateGPoint.captured ) {
                updateGPoint.captured = true;
                pointsList.captureCount++;
            } else if ( !isCaptured && updateGPoint.captured ) {
                updateGPoint.captured = false;
                pointsList.captureCount--;
                if ( pointsList.captureCount < 0 ) {
                    pointsList.captureCount = 0;
                    $.console.warn('updatePointerCaptured() - pointsList.captureCount went negative');
                }
            }
        } else {
            $.console.warn('updatePointerCaptured() called on untracked pointer');
        }
    }
Sets or resets the captured property on the tracked pointer matching the passed gPoint's id/type

@function
@private
@inner
@param {OpenSeadragon.MouseTracker} tracker
    A reference to the MouseTracker instance.
@param {Object} gPoint
    An object with id and type properties describing the pointer to update.
@param {Boolean} isCaptured
     Value to set the captured property to.