extracting a portion of a GdkPixmap

["Boncek, John" <jboncek Hunter COM>]

I have a GdkPixmap.  I need to programmatically extract a new GdkPixmap
from this image based on x- and y-coordinates, plus width and height.  The
x, y, width, and height parameters will vary too much at runtime to store
all the copies needed.  Conceptually this seems simple.  But from among
the enormous number of different image formats and image-manipulation
functions in GTK/GDK, I can't find a way to do it, after a *lot* of
searching.