GTK+ back buffers?

[Pete Ryland <pdr-lists pdr ml org>]

Hi,

I'm trying to do some fast graphical stuff, and I need to implement a fast
off-screen double buffer.  Since I also need portability with other
platforms, the backbuffer is an "unsigned char *" and can be configured to
be either 16 or 24 bit (or 8 bit with a certain fixed palette - not really
applicable in X since you can't change the whole palette). 

My question is this:  How do I copy the "unsigned char *" buffer to a
format that can be flipped to a window without having to do a put_pixel
call for each pixel in the image?

thx in advance,
Pete