This window provides double buffering and also the ability to draw the
"overlay" which is another picture placed on top of the main image. The
overlay is designed to be a rapidly-changing but simple graphic such as
a mouse selection box. Fl_Overlay_Window uses the overlay
planes provided by your graphics hardware if they are available.
If no hardware support is found the overlay is simulated by drawing
directly into the on-screen copy of the double-buffered window, and
"erased" by copying the backbuffer over it again. This means the
overlay will blink if you change the image in the window.
Creates a new Fl_Overlay_Window widget using the given
position, size, and label (title) string.
Destroys the window and all child widgets.
You must subclass Fl_Overlay_Window and provide this method.
It is just like a draw() method, except it draws the overlay.
The overlay will have already been "cleared" when this is called. You
can use any of the routines described in
Call this to indicate that the overlay data has changed and needs to
be redrawn. The overlay will be clear until the first time this is
called, so if you want an initial display you must call this after