I wrote this quite a while ago, but figured I'd post it for interweb indexing and searchability. The example is a Gtk.Window with an RGBA colormap and no window decoration. Compiz will not shadow these windows, so sometimes you want to draw your own shadow and perhaps add rounded corners.
The guassian algorithm comes from patches by Ubuntu for libgksu and therefore is licensed under the LGPL-2.
This was written before libdecoration was really usable. That might be a better way to go these days. However, maybe not, I haven't used it.