构造一个Camera类，它代表着摄像机，我们应该提供对摄像机的一般控制操作，例如移动，旋转，设置坐标，获取坐标等。这样我们就可以很容易的用鼠标键盘控制Camera进行漫游世界-Construct a Camera class that represents the camera, we should provide the general control of the camera operations, such as mobile, rotating, set coordinates, su