unity, OnTriggerStay/OnTriggerStay2D not called every fixedUpdate frame
ref: http://answers.unity3d.com/questions/1268607/ontriggerstay2d-do-not-called-every-fixedupdate-un.html#answer-1268627
ref: http://answers.unity3d.com/questions/1268607/ontriggerstay2d-do-not-called-every-fixedupdate-un.html#answer-1268627