fix #289952 : gliss. text is misplaced on exporting the score

[AntonioBL]

Resolves: https://musescore.org/en/node/289952

The "gliss." text width was computed by assuming screen metrics, while exporting (as pdf, svg or png) assumes a DPI value usually different from the screen one.

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

[AntonioBL]

I am expecting that "gliss-1" vtest should improve with this PR.
(That's why I didn't add an additional vtest)

[AntonioBL]

Note: I was misled by Qt documentation: this function https://doc.qt.io/qt-5/qfontmetrics.html#QFontMetrics-2 was not introduced in Qt 5.13 (as stated in the docs). In Qt 5.13 the second argument simply changed to const, but the function was present even before.

[AntonioBL]

As an example, gliss-1:
Before

After

Diff

[worldwideweary]

What's up with the CI_vtests/run_vtests error?

[AntonioBL]

That's because the behavior of the rendering of some vtests has changed compared to the parent branch. In fact, the positioning with this PR has improved (the "gliss." position is now the same as seen on the screen).