How to read window pixels ?

[Evonia Perrez <evonia822 gmail com>]

How to read the pixel values of a Gtk window from within the application
that created the window, such as for saving an image of the window ?  I
realize that a user could use another application, depending on the
platform, to take a snapshot of the window.  But how to take a snapshot
from within the C Gtk application without forcing the user to use another
tool ?

I have found calls for reading image files and displaying them within
Gtk2+.
But I have not been able to find any calls for saving the window as an
image.

If I could just access the RGB values of the window's pixels, then I could
handle the rest.
It looks like there might be a call to get pixels from cairo drawables, but
it would be better get the pixels from all parts of a Gtk window.