VDisplayOverlayPoints - displays 1-bit overlay points.
  VDisplayOverlayPoints(win, ovl, points, npoints)
  Window win;
  int ovl;
  XPoint *points;
  int npoints;
  VDisplayOverlayPoints displays 1-bit overlay points (pixels), in the image 
window or one of its subwindows. VTurnOnOverlay should be called earlier to 
turn on the overlays.
Return Value
  0 - work successfully.
  1 - memory allocation fault.
  6 - invalid window ID.
  225 - no overlay created.
  237 - the overlay does not turn on.
  win - the image window ID or one of its subwindows' ID.
  ovl - specifies the overlay number to be used to display the points.
  points - specifies a pointer to an array of XPoint structures containing
        the positions of the points.
  npoints - specifies the number of points to be displayed.
Side Effects
Entry Conditions
  VTurnOnOverlay should be called first. If ovl or npoint is not valid, or 
points is NULL, or VCreateColormap is not called earlier, or the points to be 
displayed are totally out of the specified window boundary, 
VDisplayOverlayPoints prints an error message to the standard error stream, 
produces a core dump file and exits from the current process.
Related Functions
VEraseOverlayPoints, VTurnOnOverlay, VTurnOffOverlay.