实现了基本的DDA直线、中点直线、Bresenham直线算法，实现中点画圆，能画多边形，能进行递归种子，扫面直线填充，能改变颜色和画笔大小。-DDA to achieve the basic line, the midpoint of a straight line, Bresenham linear algorithm, the midpoint of the circle, to draw a polygon, the seeds can be recursive, linear sweep