VDisplayOverlay - displays a 1-bit overlay image.
  VDisplayOverlay(win, data, xloc, yloc, width, height, ovl)
  Window win;
  unsigned char *data;
  int xloc, yloc;
  int width, height;
  int ovl;
  VDisplayOverlay displays a 1-bit image within a rectangle in the image
window or one of its subwindows. The data that is passed to VDisplayOverlay 
should be packed into 1 bit-per-pixel string from the most significant to the 
least significant bit for each byte. VTurnOnOverlay should be called to turn 
on the overlay before VDisplayOverlay is called.
Return Value
  0 - work successfully.
  6 - invalid window ID.
  225 - no overlay created.
  237 - the overlay is not on.
  240 - image displayed is out of window boundary.
  win - the image window ID or one of its subwindows' ID.
  data - specifies an array of data to be displayed in the image window.
  xloc, yloc - specifies x and y coordinates of the upper left corner of the
        rectangle to the origin of the image window.
  width, height - specifies the width and height in pixels of the overlay
  ovl - specifies the overlay number to be displayed as a 1-bit image.
Side Effects
Entry Conditions
  VturnOnOverlay should be called first. If ovl, xloc, yloc, width, or height 
is invalid, or data is NULL, or VCreateColormap is not called earlier, or the 
overlay image to be displayed is totally out of the specified window boundary,
VDisplayOverlay prints an error message to the standard error stream, produces
a core dump file, and exits from the current process.
Related Functions
VDisplayColorImage, VDisplayGrayImage, VEraseOverlay, VTurnOnOverlay, VTurnOffOverlay, VPackByteToBit.