判断对象是否超出屏幕

//超出屏幕移除该对象
        offScreenRemove:function(){
            var hw = this.w * 0.5,
                hh = this.h * 0.5;
            if(this.x < -hw || this.x >  this.owner.cvs.offsetWidth + hw || this.y < -hh || this.y > this.owner.cvs.offsetHeight + hh){
                this.owner.removeChild(this);
            }
        },