实现了鼠标画图功能： 左键点击，在客户区出现一个图形标记，按住左键拖动，则可绘制图形，双击右键，清除客户区的所有图形。-Implementation of the mouse drawing functions: left click in the client area of a graphic mark, hold down the left drag, you can draw graphics, double-click the right to remove all the gra