VDisplayOverlayPoints - displays 1-bit overlay points.
VDisplayOverlayPoints(win, ovl, points, 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.
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.
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.