How to create transparent GdkPixmaps?

[Hongli Lai <h lai chello nl>]

How do I create a transparent GdkPixmap? When I use gdk_pixmap_new(), I
get a black pixmap, a pixmap containing random data or a pixmap that
contains part of another pixmap.
I already tried using XRenderCreatePicture, but a gdk_pixmap_lookup() on
the returned Picture variable fails and returns NULL, and
gdk_pixmap_foreign_new() makes the app crash (invalid pixmap or bad
match or something).
Is it possible to create a transparent pixmap? I'm trying to create a
transparent PNG file with antialiased text.