Dispose SequenceSet when we're done with it.

Utility rules that do something on a map and exit
are left without explicit disposing, as they will
be cleaned up immediately anyway.

OpenRA.Game/Map/Map.cs

@@ -149,7 +149,7 @@ namespace OpenRA
 		}
 	}
 
-	public class Map : IReadOnlyFileSystem
+	public class Map : IReadOnlyFileSystem, IDisposable
 	{
 		public const int SupportedMapFormat = 11;
 		public const int CurrentMapFormat = 12;
@@ -1417,5 +1417,10 @@ namespace OpenRA
 
 			return modData.Translation.GetString(key, args);
 		}
+
+		public void Dispose()
+		{
+			Sequences.Dispose();
+		}
 	}
 }