[LayaAirIDE 2.0]被遮罩的显示对象的子显示对象再添加遮罩，则子显示对象显示不正确

 //这段注释掉，imgMask则正常显示。

        let bigMask:Laya.Sprite = new Laya.Sprite();

        bigMask.graphics.drawRect(0, 0, 300, 200, "#ffffff");

        this.mask = bigMask;

        //



        let imgMask:Laya.Sprite = new Laya.Sprite();

        imgMask.graphics.drawRect(0, 0, 100, 300, "#ffffff");

        this.img.mask = imgMask;

        Laya.Tween.to(this.img, {x:100}, 10000).update = Laya.Handler.create(this, this.updateMsk, null, false);

    }



    private updateMsk():void

    {

        this.img.mask.graphics.clear(true);

        this.img.mask.graphics.drawRect(100 - this.img.x, 0, this.img.width, 300, "#ffffff");

    }