Expose some storyboard pieces to allow better testability

osu.Game/Storyboards/Drawables/DrawableStoryboard.cs

@@ -23,7 +23,9 @@ namespace osu.Game.Storyboards.Drawables
 {
     public partial class DrawableStoryboard : Container<DrawableStoryboardLayer>
     {
-        [Cached]
+        public Vector2 AppliedScale { get; private set; }
+
+        [Cached(typeof(Storyboard))]
         public Storyboard Storyboard { get; }
 
         /// <summary>

osu.Game/Storyboards/Drawables/DrawableStoryboardLayer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System.Collections.Generic;
 using System.Threading;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
@@ -30,10 +31,12 @@ public DrawableStoryboardLayer(StoryboardLayer layer)
             InternalChild = ElementContainer = new LayerElementContainer(layer);
         }
 
-        protected partial class LayerElementContainer : LifetimeManagementContainer
+        public partial class LayerElementContainer : LifetimeManagementContainer
         {
             private readonly StoryboardLayer storyboardLayer;
 
+            public IEnumerable<Drawable> Elements => InternalChildren;
+
             public LayerElementContainer(StoryboardLayer layer)
             {
                 storyboardLayer = layer;

osu.Game/Storyboards/Storyboard.cs

@@ -86,7 +86,7 @@ public bool ReplacesBackground
             }
         }
 
-        public DrawableStoryboard CreateDrawable(IReadOnlyList<Mod>? mods = null) =>
+        public virtual DrawableStoryboard CreateDrawable(IReadOnlyList<Mod>? mods = null) =>
             new DrawableStoryboard(this, mods);
 
         private static readonly string[] image_extensions = { @".png", @".jpg" };

osu.Game/Storyboards/StoryboardVideo.cs

@@ -14,7 +14,7 @@ public class StoryboardVideo : IStoryboardElement
 
         public double StartTime { get; }
 
-        public StoryboardVideo(string path, int offset)
+        public StoryboardVideo(string path, double offset)
         {
             Path = path;
             StartTime = offset;