GetModeRange function

  DECLARATION:  GetModeRange(GraphDriver : Integer;
                             var LoMode, HiMode : Integer);

      PURPOSE:  Returns the range of valid graphic modes for a driver

         UNIT:  Graph

      REMARKS:  LoMode will contain the lowest value and HiMode will contain
                the highest mode available for the specified graphics driver.

                NOTES:  If GraphDriver is invalid, LoMode and HighMode will
                        be set to -1.  See the table under GetGraphMode.