/* $DOC$
   $NAME$
      ft_Shadow()
   $CATEGORY$
      Video
   $ONELINER$
      Draw a non-destructive shadow on the screen
   $SYNTAX$
      ft_Shadow( <nTop>, <nLeft>, <nBottom>, <nRight> [ ,<nAttr>] ) -> NIL
   $ARGUMENTS$
      <nTop>    is the top row of the shadow area.
      <nLeft>   is the upper left column of the shadow area.
      <nBottom> is the bottom row of the shadow area.
      <nRight>  is the lower right column of the shadow area.
      <nAttr>   is optional and is the screen attribute to use for drawing
                the shadow.  If not specified, the default is 8.
   $RETURNS$
      NIL
   $DESCRIPTION$
      This function allows you to implement the popular "shadow effect."  It
      draws a shadow using the specified screen coordinates.  The shadow
      is drawn along the bottom and right side of the specified region.

      The source code is written to TASM IDEAL mode.
   $EXAMPLES$
      ft_Shadow( 10, 10, 15, 50, 8 )   // draw a dim shadow

      ft_Shadow( 10, 10, 15, 40, 47 )  // draw a green shadow
   $END$
 */