Dispose MapPreviews and map packages.

OpenRA.Game/Map/MapPreview.cs

@@ -53,7 +53,7 @@ namespace OpenRA
 		public readonly bool downloading;
 	}
 
-	public class MapPreview
+	public class MapPreview : IDisposable
 	{
 		static readonly CPos[] NoSpawns = new CPos[] { };
 		MapCache cache;
@@ -344,5 +344,14 @@ namespace OpenRA
 		{
 			Status = MapStatus.Unavailable;
 		}
+
+		public void Dispose()
+		{
+			if (Package != null)
+			{
+				Package.Dispose();
+				Package = null;
+			}
+		}
 	}
 }