cheap partial fix for cross-system glissandi

Very partial fix; it just draws a short gliss into the second note.  So nothing drawn coming out of the first note.  Ultimately we need an implementation where the gliss is divided into segments, perhaps one that also allows editing the length and/or angle of the gliss.  But if we don't do that for 2.0, I think we should do as I do in this PR.  It's not good, but it's much better than what we do currently.