[MU4 Issue] Insufficient control over portamento playback for glissandi

Describe the bug
In vocal and unfretted string music in particular, glissandi playback should a) be portamento by default b) allow fine-tuning of when the portamento starts

To Reproduce
Steps to reproduce the behavior:

1. Create a new score using singing voices or unfretted string instruments
2. Add two notes
3. Add a glissando between them
4. Play it back: doesn't sound like any glissando I'd ever hear a singer or orchestral string player use
5. Change the playback mode to "portamento" and play it back again. While not incorrect, it sounds unmusical due to there being no time allowed to establish the pitch of the first note.
6. Adjust the start position of the glissando so it appears half-way through the note. Playback is unchanged.
7. Look for some other way to control glissando playback...can't find any!

Expected behavior
For appropriate instruments (human voices, unfretted strings, trombone etc.) the default glissando mode should be portamento.
Either the default portamento playback should allow some "delay" to establish the pitch before starting the portamento, or have an option to control it (as there are cases no such delay is intended by the composer).

Screenshots
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Platform information

• Windows 11

Additional context
Using a tie doesn't help either, as glissandi/portamenti don't play starting from tied notes.
NB for "piano" the default playback for glissandi is still "chromatic" despite it being impossible to play a chromatic glissando on any piano I've ever seen. And there's a limit to how slowly a glissando can feasibly be played which MuseScore doesn't seem to worry about.

No source files, tests, or entry points are named. Start by reproducing the glissando playback cases described on Windows 11, including portamento mode and changing the glissando start position. Done means appropriate instruments default to portamento and playback provides a controllable pitch-establishment delay without breaking the piano and tied-note cases.