DBE incompatible with Xrender?

[Po Lu]

I can't seem to draw to a Picture corresponding to a back buffer created
by the double buffer extension; when I swap the window with XdbeCopied
it becomes apparent that no drawing has actually occured, but no error
is generated either, and it works fine if I create the picture for the
window and not the back buffer (and avoid swapping buffers, of course).

The picture format I'm using to create the picture is the picture format
returned by XRenderFindVisualFormat for the same visual that created the
window the back buffer was allocated for.

I must be missing something here.  Any ideas?

Thanks in advance.