说明：  利用opengl实现中点Bressonham画直线和圆，改进的Bressonham画直线等。容计算机画图算法和opengl应用技术为一体的代码，非常值得大家学习！
(Opengl midpoint Bressonham realize the use of a straight line and circle drawing, improved Bressonham such as painting a straight line. Let the computer opengl drawing algorithms and application technology as one of the code, everyone is very worthy of study!)