multitouch point don't update if drag out

1. touch P1 inside MultiPointTouchArea
2. keep P1 touch
3. touch P2 outside MultiPointTouchArea
4. drag P2 into MultiPointTouchArea
5. drag P2 out of MultiPointTouchArea

point of P2 will stop at the border of MultiPointTouchArea

expect P2 to get updated even dragged out of MultiPointTouchArea

reproducible example:
multitouchtest.zip