openGL中的变换过程，理解透视投影与平行投影的不同，添加代码实现三种场景中的物体三维模型，通过变换调整观察的位置与方向-openGL in the transformation process and understand the perspective projection and parallel projection differences, add the code to achieve the three kinds of objects in the scene three-dimensional model, through the transformation to adjust the position and direction of observation