Graphics::Clear(Color::Transparent);实际不会生效。该函数相当于在原有的基础上叠加一层，而叠加透明色的结果就是原来的图片。
参考文献：
1.http://stackoverflow.com/questions/26224018/c-sharp-graphics-clearcolor-transparent-not-working-properly
2.http://stackoverflow.com/questions/1609032/how-to-clear-graphics-on-a-transparent-control-in-windows-forms
3.https://social.msdn.microsoft.com/Forums/en-US/17bd0d8e-ece1-4bee-a926-fe67bcdd4081/how-to-use-graphicsclear?forum=csharplanguage
4.http://www.dreamincode.net/forums/topic/87804-transparent-color-is-black/