Fix storyboard videos not displaying

Regressed with 102085668f because the
stupid magic alpha transform addition was also implicitly changing the
value of `IsDrawable` from false to true because that property checks
for presence of any commands.

Apparently past me, in his infinite wisdom, did not decide it pertinent
to test that change against, you know, *a beatmap with a storyboard*.
Great job, past me, good show all around.

osu.Game/Storyboards/StoryboardSprite.cs

@@ -17,7 +17,7 @@ namespace osu.Game.Storyboards
         private readonly List<StoryboardTriggerGroup> triggerGroups = new List<StoryboardTriggerGroup>();
 
         public string Path { get; }
-        public bool IsDrawable => HasCommands;
+        public virtual bool IsDrawable => HasCommands;
 
         public Anchor Origin;
         public Vector2 InitialPosition;

osu.Game/Storyboards/StoryboardVideo.cs

@@ -19,6 +19,8 @@ namespace osu.Game.Storyboards
 
         public override double StartTime { get; }
 
+        public override bool IsDrawable => true;
+
         public override Drawable CreateDrawable() => new DrawableStoryboardVideo(this);
     }
 }