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, such as access to the coordinates. So that we can easily control the keyboard with the mouse roaming Camera World