Graphics::Clear(Color::Transparent);實際不會生效。該函數相當於在原有的基礎上疊加一層，而疊加透明色的結果就是原來的圖片。
參考文獻：
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/