Fix lead-in logic to match stable

Also adds storyboard event priority support.

osu.Game/Screens/Play/GameplayClock.cs

@@ -34,7 +34,6 @@ public GameplayClock(IFrameBasedClock underlyingClock)
         public void ProcessFrame()
         {
             // we do not want to process the underlying clock.
-            // this is handled by PauseContainer.
         }
 
         public double ElapsedFrameTime => underlyingClock.ElapsedFrameTime;

osu.Game/Screens/Play/GameplayClockContainer.cs

@@ -99,7 +99,13 @@ private void load(OsuConfigManager config)
             userAudioOffset = config.GetBindable<double>(OsuSetting.AudioOffset);
             userAudioOffset.BindValueChanged(offset => userOffsetClock.Offset = offset.NewValue, true);
 
-            adjustableClock.Seek(Math.Min(0, gameplayStartTime - totalOffset - (allowLeadIn ? beatmap.BeatmapInfo.AudioLeadIn : 0)));
+            double startTime = -beatmap.BeatmapInfo.AudioLeadIn;
+
+            startTime = Math.Min(startTime, beatmap.Storyboard.FirstEventTime);
+            startTime = Math.Min(startTime, gameplayStartTime);
+            startTime = Math.Min(startTime, 0);
+
+            Seek(startTime);
             adjustableClock.ProcessFrame();
 
             UserPlaybackRate.ValueChanged += _ => updateRate();

osu.Game/Storyboards/Storyboard.cs

@@ -17,6 +17,8 @@ public class Storyboard
 
         public bool HasDrawable => Layers.Any(l => l.Elements.Any(e => e.IsDrawable));
 
+        public double FirstEventTime => Layers.Min(l => l.Elements.FirstOrDefault()?.StartTime ?? 0);
+
         public Storyboard()
         {
             layers.Add("Background", new StoryboardLayer("Background", 3));