图形学实验，用opengl画圆，实现中点Bresenham算法画直线，实现改进Bresenham算法画直线码，实现具有宽度的图形（线刷子或方刷子）-Graphics experiment, using opengl draw a circle to achieve the midpoint of Bresenham algorithm for drawing a straight line, to achieve improved Bresenham algorithm for drawing