VTurnOnOverlay - turns the specified overlay on.
  VTurnOnOverlay(ovl, red, green, blue)
  int ovl;
  unsigned short red, green, blue;
  VTurnOnOverlay turns the specified overlay on with the specified RGB color.
VTurnOnOverlay changes the colormap entries and updates the 3DVIEWNIX
reserved color for the specified overlay. VTurnOnOverlay should be called
before calling other overlay functions, except VChangeNumberOfOverlays.
Return Value
  0 - work successfully.
  1 - memory allocation error.
  225 - no overlay created.
  ovl - specifies the overlay number to be turned on.
  red, green, blue - the RGB color to be used for the overlay.
Side Effects
Entry Conditions
  If ovl is not valid, or VCreateColormap is not called earlier,
VTurnOnOverlay prints an error message to the standard error stream,
produces a core dump file and exits from the current process.
Related Functions
VChangeNumberOfOverlays, VTurnOffOverlay.