说明：  c3DS类负责读取3ds文件结构，它提供一系列的虚函数来读数据，但并不保存数据，你必须自己写一个类来继承它，并覆盖这些虚函数。在具体例程中我写了一个叫做c3DSMODEL的类，它只读网格数据，并渲染。
(Class responsible for c3DS read 3ds file structure, which provides a series of virtual function to read data, but it does not save data, you have to write a class to inherit it and override these virtual functions. Specific routines, I wrote a class called c3DSMODEL, it read-only grid data, and rendering.)