You wrote:
However, I also tried it with Luís' new-types branch, and that fails with a different error now, something about rectangles not being SAPs.
http://article.gmane.org/gmane.lisp.cffi.devel/1034
Of course, since the new-types branch is apparently still experimental, that might not be an issue in graphic-forms at all.
Thanks for reporting this, too. I haven't yet figured out what to do about it but I'm still investigating.
I guess it's time to resubscribe to a couple mailing lists :-)