用Bresenham画线算法实现水平、垂直、斜率大于1、斜率小于1、斜率为正、斜率为负等各种情况（不能直接调用OpenGL画线函数）。(Using Bresenham line drawing algorithm to achieve horizontal, vertical, slope greater than 1, slope is less than 1, slope is positive, slope is negative, and so on, and can not directly call OpenGL line function.)