当鼠标左键点下时，固定画线的起点，鼠标移动，线段跟找移动，左键再点下时画出线段，具有保存原来画的线段的功能-When the next point when the left mouse button, fixed the starting point for drawing a line, mouse, mobile, find a line segment with the mobile, left again when the draw under the line segment, wi