用纯windows api编写的画线小程序，但是通过动态内存分配能保存所画的所有的线，同时通过图像双缓冲技术使得图像的显示非常流畅，另外运用了映射模式实现了就算是所画的线超出了窗口的范围，也能在窗口中显示出来的功能。-Written in pure windows api to draw a line applet, but dynamic memory allocation can save all the lines drawn, and through the double-bufferi