For the sake of reference: The source of the problem seems to have
been identified:
http://lists.gnu.org/archive/html/lilypond-devel/2014-02/msg00238.html
So does this need a LilyPond Tracker
Yes, definitely.
or is this a bug (in fontforge?)?
It's basically a bug in fontforge, but we can't rely on a quick fix
since this is very unlikely. Additionally, we rely on the last
*published* fontforge version, which was a released quite a long time
ago. Meanwhile, fontforge has been very heavily modified.
Unfortunately, not all modifications are bug free; for example, I
can't build the feta fonts with current fontforge's current git
version due to a very fundamental bug, cf.
https://github.com/fontforge/fontforge/issues/1239
The solution to the clef problem is not too difficult, however: We
have to slightly modify the outlines of the clefs so that the
intersections are `simpler', allowing fontforge to handle the outline
correctly. While reviewing Marc's patch I've only checked one size
(feta20), which was an oversight.
Marc, do you have time to work on that?