Add original sprite and visualiser

Notes: This is using a modified version of welcome.osz to facilitate the visualiser and the animation of the sprite is not accurate.

osu.Game/Screens/Loader.cs

@@ -52,7 +52,7 @@ namespace osu.Game.Screens
                     return new IntroCircles();
 
                 case IntroSequence.Welcome:
-                    return new IntroLegacy();
+                    return new IntroWelcome();
 
                 default:
                     return new IntroTriangles();