tw_VAttr( wT, wL, wB, wR, pattr )
 ----------------------------------------------------------------------------
     Change the colour in a screen region to that defined by pattr
     without disturbing the contents of the screen.

       Arguments:

     wT,wL
     wB,wR   - 'N' Screen Coordinates. This defaults to Full Screen if
                   not passed.

       Return:

     NIL

       Example:

     tw_VAttr( wT + 1, wR + 1, wB + 1, wR + _twShTh() ,;
                           IIF(ISCOLOR(),8,7))

     This is used to make one of the shadow areas in the wShadow()
     function.