VDisplayOverlayRectangle - displays a 1-bit overlay rectangle.
  VDisplayOverlayRectangle(win, ovl, xloc, yloc, width, height)
  Window win;
  int ovl;
  int xloc, yloc;
  int width, height;
  VDisplayOverlayRectangle displays a 1-bit overlay rectangle in the image
window or one of its subwindows. VTurnOnOverlay should be called to turn on 
the overlays, before calling VDisplayOverlayRectangle.
Return Value
  0 - work successfully.
  6 - invalid window ID.
  225 - no overlay created.
  237 - the overlay is not on.
  win - the image window ID or one of its subwindows' ID.
  ovl - specifies the overlay number to be used to draw the overlay rectangle.
  xloc, yloc - specifies x and y coordinates of the upper left corner of the
        rectangle, relative to the origin of the specified window.
  width, height - specifies the width and height in pixels of the frame.
Side Effects
Entry Conditions
  VTurnOnOverlay should be called first. If ovl, xloc, yloc, width, or height 
is not valid, or VCreateColormap is not called earlier, or the rectangle to 
be displayed is totally out of the specified window boundary, 
VDisplayOverlayRectangle prints an error message to the standard error stream,
produces a core dump file and exits from the current process.
Related Functions
VTurnOnOverlay, VTurnOffOverlay, VEraseOverlayRectangle.