This error appears to be related to :unicode in *features*. When :unicode is set, (setf xlib:gcontext-font) is never called on the gcontext returned from medium-gcontext. I can stuff font into the gcontext and continue the drawing operation, but error is quickly triggered again. I traced xlib:create-gcontext and found that a new context is created.
I don't really have any idea what I'm doing, but I thought I might get things to continue if i modified medium-gcontext to always call gcontext-font: (setf (xlib:gcontext-font gc) (xlib:open-font (first xlib::*displays*) "fixed"))
This allows the demo to continue without further errors, but nothing really very interesting appears in the demo window except a narrow black box at the top, with a wide white box beneath it. At this point I really don't have any idea what's wrong. Can anyone help me or show me where to start looking?