Line procedure

  DECLARATION:  Line(X1,Y1,X2,Y2 : Integer)

      PURPOSE:  Draws a line between two points

         UNIT:  Graph

      REMARKS:  Draws a line from (X1,Y2) to (X2,Y2), using the current
                line style and thickness as defined by SetLineStyle.
                The line will also be in the color as defined by SetColor.
                Use SetWriteMode to determine whether the line is copied or
                XOR'd to the screen.

 RESTRICTIONS:  Must first be in graphics mode.