SetVisualPage procedure

  DECLARATION:  SetVisualPage(PageNumber : Word);

      PURPOSE:  Sets the visual graphics page number

         UNIT:  Graph

      REMARKS:  With multiple graphic pages, you can write to, or update
                a graphic page that is not visable, then quickly display
                it by making it active - this can help with animation and
                help screens.

                Multiple graphic pages are only supported by 256K EGA, VGA
                and Hercules graphics cards.

 RESTRICTIONS:  Must first be in graphics mode.