Existing cross-staff glissando not copied when generating parts

Ubuntu 14.04, GIT commit: 699a5ee

First (?) reported in https://musescore.org/en/comment/reply/74481. There are some sort of similar-seeming reports, but I don't think any of them have the same cause.

1) load attached file
2) generate parts
3) view part

Result: glissandi do not appear. They do if you delete the glissando then add it back - only pre-existing glissandi are affected.

A couple of different problems seem to contribute to this. One of them is here:

https://github.com/musescore/MuseScore/blob/a23ef389c0f3d3a5136b1b7371c…

It works better if I reverse oldStart and oldEnd here - otherwise we are computing the wrong track number, I think. But even with this change, it still only works if we have already processed the start element, which won't be the case if it's on a lower staff in the same measure. So I get the first, second, and fourth glissandi in the attached file, but not the third. I suspect fixing that case that would require deeper changes.