MFC界面画多边形，每点一次画一个点，双击完成画多边形。可以随意拉动多边形的角，改变多边形的形状，可用其来选中界面的一部分，多边形整体也可以移动。(MFC interface draws polygon, each point draws a point at a time, double-click to complete drawing polygon. You can pull the corners of the polygon at will to change the shape o