Find video by recursive check rather than marker interface

Seems a lot more reliable, and allows falling back to the "main" video
in cases which didn't support this previously.

A next step may be to allow every screen to support a video based on its
screen name, rather than specifying the local `TourneyVideo` every time.

osu.Game.Tournament/Components/TourneyVideo.cs

@@ -22,6 +22,8 @@ namespace osu.Game.Tournament.Components
         private Video video;
         private ManualClock manualClock;
 
+        public bool VideoAvailable => video != null;
+
         public TourneyVideo(string filename, bool drawFallbackGradient = false)
         {
             this.filename = filename;