fontchooser: hexboxes in preview
We disable font fallback in the fontchooser preview, which makes sense.
But, it also affects preedit text when using input methods. And, nowadays IBus (or gnome-shell, via the text protocol), sends us things like ⎄ as part of preedit, which yields ugly results:
I think we should unconditionally enable fallback for preedit, since we really don't control what shows up there.