Ignore zip files in level metadata loading

Zip files that have been successfully mounted in editorclass::loadZips()
will now be ignored when the game does its second pass over the levels
directory. Otherwise, this would produce a superfluous error message,
because the game would attempt to parse the zip file as a level file
(when it's not a level file and is in fact a binary file).

desktop_version/src/editor.cpp

@@ -206,7 +206,7 @@ static void levelMetaDataCallback(const char* filename)
     LevelMetaData temp;
     std::string filename_ = filename;
 
-    if (!FILESYSTEM_isFile(filename))
+    if (!FILESYSTEM_isFile(filename) || FILESYSTEM_isMounted(filename))
     {
         return;
     }