GetImage procedure

  DECLARATION:  GetImage(x1, y1, x2, y2 : Integer; var BitMap)

      PURPOSE:  Saves a bit image of the specified area into a buffer

         UNIT:  Graph

      REMARKS:  x1, y1, x2, and y2 define a rectangular portion of the
                screen. BitMap is an untyped parameter which must be greater
                than or equal to 4 plus the amount of area defined by the
                region.

                The first two words of BitMap are reserved for the height and
                width of the region.  The remainder of BitMap is used to save
                the bit image.  The ImageSize function can be used to
                determine the size requirements of BitMap.

 RESTRICTIONS:  Must be in graphics mode.