Fix multiplayer spectator potentially taking too long to start

When watching from the middle of gameplay, due to a series of failures,
`SpectatorClock` would not get seeked to the current time, causing all
clients to look like they were out of sync.

This is a hotfix for the issue. A better fix will require framework
changes or considerable restructuring.

I'd recommend testing this works in practice and agreeing that while it
is a hack, it's likely not going to cause issues and is something we
want to see fixed sooner rather than later.

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/SpectatorPlayerClock.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using osu.Framework.Logging;
 using osu.Framework.Timing;
 using osu.Game.Screens.Play;
 
@@ -59,6 +60,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
 
         public bool Seek(double position)
         {
+            Logger.Log($"{nameof(SpectatorPlayerClock)} seeked to {position}");
             CurrentTime = position;
             return true;
         }

osu.Game/Screens/Play/GameplayClockContainer.cs

@@ -160,6 +160,13 @@ namespace osu.Game.Screens.Play
 
             Seek(StartTime);
 
+            // This is a workaround for the fact that DecoupleableInterpolatingFramedClock doesn't seek the source
+            // if the source is not IsRunning. (see https://github.com/ppy/osu-framework/blob/2102638056dfcf85d21b4d85266d53b5dd018767/osu.Framework/Timing/DecoupleableInterpolatingFramedClock.cs#L209-L210)
+            //
+            // This breaks in multiplayer spectator.
+            // I hope to remove this once we knock some sense into clocks in general.
+            (SourceClock as IAdjustableClock)?.Seek(StartTime);
+
             if (!wasPaused || startClock)
                 Start();
         }