Don't load project twice when converting parts

[cbjeukendrup]

and fix a bug where the ret of convertScorePartsToPdf etc. was ignored

[cbjeukendrup]

@DmitryArefiev Please check if there are no regressions with --export-score-parts command line option

[DmitryArefiev]

@DmitryArefiev Please check if there are no regressions with --export-score-parts command line option

@cbjeukendrup Should be checked on all OSs?

[cbjeukendrup]

One should be enough :)

[DmitryArefiev]

@cbjeukendrup From https://musescore.org/en/handbook/3/command-line-options I see:

-P | --export-score-parts

When converting to PDF with the -o option, append each part’s pages to the created PDF file. If the score has no parts, all default parts will temporarily be generated automatically.

Can you help with syntax please?

I know how it works with -o command when converting to pdf (on Windows):

musescore.exe -o [filename].pdf [filename].mscz

but didn't find how to add -P command

[DmitryArefiev]

@cbjeukendrup OK. I've found the way:

MuseScore.exe --export-score-parts 1.mscz -o 1.pdf

(-P doesn't work with MuseScore4)

@DmitryArefiev Please check if there are no regressions with --export-score-parts command line option

There is no regressions with master. But --export-score-parts + -o commands don't work properly with 4.6 master as it work with MuseScore3. Is it expected?

[cbjeukendrup]

I wasn't fully aware it was broken, but it seems there is indeed an open issue about it: #22887
Anyway, I'll merge this PR, because it fixes a little bit of tech debt, and then we can fix that issue later, depending on how it is prioritised.