VDisplayOverlayLine - displays a 1-bit overlay line.
  VDisplayOverlayLine(win, ovl, x1, y1, x2, y2)
  Window win;
  int ovl;
  int x1, y1, x2, y2;
  VDisplayOverlayLine displays a 1-bit overlay line in the image window or
one of its subwindows. VTurnOnOverlay should be called to turn on the 
overlays before calling VDisplayOverlayLine.
Return Value
  0 - work successfully.
  1 - memory allocation error.
  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 display the line.
  x1, y1 - specifies the first point of the line.
  x2, y2 - specifies the second point of the line.
Side Effects
Entry Conditions
  VTurnOnOverlay should be called first. If ovl, x1, x2, y1, or y2 is not 
valid, or VCreateColormap is not called earlier, or the bounding box of the 
line segment to be displayed is totally out of the specified window boundary, 
VDisplayOverlayLine prints an error message to the standard error stream, 
produces a core dump file and exits from the current process.
Related Functions
VEraseOverlayLine, VTurnOnOverlay, VTurnOffOverlay.