twLineShad( bRows, bCols, Buttons, tShadow )
 ----------------------------------------------------------------------------
     Create a half height, half width shadow around an area in a
     window. This function is used in the alert class and in
     twCreateButton().

       Arguments:

     wT, wL  - 'N' Relative Window Coordinates

     cButton - 'C' String to put on the button in order to get the
                   length.

     wColor  - 'C' Clipper colour attribute. Defaults to current window
                   color.

     wSType  - 'N' TSDWIN Shadow Type. Defaults to current shadow type.

       Return:

     NIL

       Example:

     twLineShad( 5, 5, "Trial", 3 )