Expose the latest end time of storyboard elements

Co-authored-by: Marlina Bowring <marlina@umich.edu>

osu.Game/Storyboards/IStoryboardElement.cs

@@ -12,6 +12,8 @@ namespace osu.Game.Storyboards
 
         double StartTime { get; }
 
+        double EndTime { get; }
+
         Drawable CreateDrawable();
     }
 }

osu.Game/Storyboards/Storyboard.cs

@@ -36,6 +36,15 @@ namespace osu.Game.Storyboards
         /// </remarks>
         public double? EarliestEventTime => Layers.SelectMany(l => l.Elements).OrderBy(e => e.StartTime).FirstOrDefault()?.StartTime;
 
+        /// <summary>
+        /// Across all layers, find the latest point in time that a storyboard element ends at.
+        /// Will return null if there are no elements.
+        /// </summary>
+        /// <remarks>
+        /// This iterates all elements and as such should be used sparingly or stored locally.
+        /// Videos and samples return StartTime as their EndTIme.
+        /// </remarks>
+        public double? LatestEventTime => Layers.SelectMany(l => l.Elements).OrderByDescending(e => e.EndTime).FirstOrDefault()?.EndTime;
         /// <summary>
         /// Depth of the currently front-most storyboard layer, excluding the overlay layer.
         /// </summary>

osu.Game/Storyboards/StoryboardSample.cs

@@ -15,6 +15,8 @@ namespace osu.Game.Storyboards
 
         public double StartTime { get; }
 
+        public double EndTime => StartTime;
+
         public int Volume { get; }
 
         public IEnumerable<string> LookupNames => new[]

osu.Game/Storyboards/StoryboardVideo.cs

@@ -14,6 +14,8 @@ namespace osu.Game.Storyboards
 
         public double StartTime { get; }
 
+        public double EndTime => StartTime;
+
         public StoryboardVideo(string path, int offset)
         {
             Path = path;