VFillOverlayRectangle - fills a 1-bit overlay rectangle.
  VFillOverlayRectangle(win, ovl, xloc, yloc, width, height)
  Window win;
  int ovl;
  int xloc, yloc;
  int width, height;
  VFillOverlayRectangle fills a 1-bit overlay rectangle in the image window
or one of its subwindows. If the width and height are equal to zero, 
VFillOverlayRectangle fills the whole specified window from the specified 
location. If the image is partially out of the window boundary, 
VFillOverlayRectangle clips the image, displays the clipped overlay image in 
the window, and returns an error code.
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 the window boundary.
  ovl - the overlay number.
  xloc,yloc - specifies x and y coordinates of the upper left corner of the
        rectangle, relative to the origin of the image window.
  width,height - specifies the width and height in pixels of the overlay.
Side Effects
Entry Conditions
  VTurnOnOverlay should be called first. If ovl, xloc, yloc, width, or height 
is invalid, or VCreateColormap is not called earlier, or the overlay image to 
be filled is totally out of the specified window boundary, 
VFillOverlayRectangle prints an error message to the standard error stream, 
produces a core dump file and exits from the current process.
Related Functions
VDisplayOverlay, VDisplayOverlayRectangle.