fix #38501: cheap partial fix for cross-staff and cross-system gliss

Addresses both http://musescore.org/en/node/38501 and http://musescore.org/en/node/3811 (replacing an earlier closed PR for the latter).  It's only a partial fix for both issues, as you can see from the vtests, but it's a heck of a lot better than what we do now.  There has been talk of updagting the glissando implementation in a way that would allow for a full solution, but its kind of looking less and less likely that we'll see that for 2.0, so I would like to have this as a fallback.