this is base class of all 'top' font table
 public bool TryGetTable(string tableName, out TableEntry entry)
 {
     return _tables.TryGetValue(tableName, out entry);
 }
 public void ReplaceTable(TableEntry table)
 {
     _tables[table.Name] = table;
 }
 public void AddEntry(TableEntry en)
 {
     _tables.Add(en.Name, en);
 }