编写Windows 窗口应用程序，通过鼠标拖动画图，要求在窗口切换到后台再切换到前台的时候，之前绘制的内容仍然能正确显示，类似Windows中的画笔软件-Windows window application written by dragging the mouse drawing, required to switch to the window and then switch back to the front when the draw before the content is stil