实现绘图的功能，主要画点，直线，矩形，椭圆，可改变线型线宽，用滚动条控件实现的调色板功能，避免了窗口重绘时图像消失-Achieve the graphics function, the main drawing points, lines, rectangles, ellipses, can change the line width with the slider control to achieve the palette function, avoiding the window to r