在多媒体教学时常需在全屏上对课件进行各种重点标注，该代码实现屏幕绘图功能，并具有利用快捷键来改变画笔颜色、宽度等功能。-In the multimedia teaching often need to the courseware in full screen on all key mark, this code realization drawing function, and has a screen using the keybo