SetActivePage procedure

  DECLARATION:  SetActivePage(Page : Word);

      PURPOSE:  Set the active page for graphics output

         UNIT:  Graph

      REMARKS:  Page becomes the active video page and all graphics output
                will be directed to Page.  To make the newly active page
                visual, use SetVisualPage.

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

 RESTRICTIONS:  Must first be in graphics mode.