Merge pull request #4938 from pchote/map-derp

Isolate gameplay map state from the map cache.

OpenRA.Game/ModData.cs

@@ -103,10 +103,12 @@ namespace OpenRA
 		{
 			LoadScreen.Display();
 
-			var map = MapCache[uid].Map;
-			if (map == null)
+			if (MapCache[uid].Status != MapStatus.Available)
 				throw new InvalidDataException("Invalid map uid: {0}".F(uid));
 
+			// Operate on a copy of the map to avoid gameplay state leaking into the cache
+			var map = new Map(MapCache[uid].Map.Path);
+
 			LoadTranslations(map);
 
 			// Reinit all our assets