Fix #313896 Portamento playdown not working

[ypoissant]

Resolves: Glissando down playback not working when set to "Portamento"

Portamento style glissando going from a higher note to a lower note did not play like a portamento but played the two begin and end note.

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• I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

I believe the commit title should be Fix #313896: Portamento playdown not working for the aute marking as fixed to work.

[ypoissant]

I just clicked on the wrong button prematurely. I edited the PR message. But I'm not sure if the direct link to the fixed issue is formatted correctly.

[Jojo-Schmitz]

That's OK, it leads to it