Dispose MapPreviews and map packages.

OpenRA.Game/Map/MapCache.cs

@@ -269,6 +269,9 @@ namespace OpenRA
 				return;
 			}
 
+			foreach (var p in previews.Values)
+				p.Dispose();
+
 			// We need to let the loader thread exit before we can dispose our sheet builder.
 			// Ideally we should dispose our resources before returning, but we don't to block waiting on the loader thread to exit.
 			// Instead, we'll queue disposal to be run once it has exited.