The Fl_Double_Window class provides a double-buffered window.
If possible this will use the X double buffering extension (Xdbe). If
not, it will draw the window data into an off-screen pixmap, and then
copy it to the on-screen window.
It is highly recommended that you put the following code before the
first show() of any window in your program:
This makes sure you can use Xdbe on servers where double buffering
does not exist for every visual.
Creates a new Fl_Double_Window widget using the given
position, size, and label (title) string.
The destructor also deletes all the children. This allows a
whole tree to be deleted at once, without having to keep a pointer to
all the children in the user code.
Returns the off-screen pixmap or back buffer. This value is zero until
the first time flush() is called.