实现DDA直线画法和中点画圆法。 实现vc++6.0下实践图形算法（要求任意指定一个起始点坐标作为圆心和终止点坐标形成的半径） -Achieve DDA law and the midpoint of a straight line drawn drawcircle. Achieve vc++ 6.0 under the practice of graphics algorithm (request arbitrary coordinates to specify a starting