Fix MSVC builds / update to Qt 5.9.9 (patched)

[Jojo-Schmitz]

MSCV compilers 16.6.3 and later (current is 16.6.5) trigger a bug in Qt code, so need patched versions of Qt 5.9 (like available at qt599_msvc2015.7z and qt599_msvc2017_64.7z) or the official Qt 5.12.9 or even Qt 5.15.0.
The former should get used for the 3.x branch, the latter might get used for the master branch (let me know if you want packages for those).

See also the previous #6294

As is (i.e. without the updated Qt packages) this PR will fail the appveyor build...

BTW: I think we should update all the other builds to (at least) Qt 5.9.9, the latest and last 5.9 patch. We should do this at least for the 3.x branch, for master branch we may want to go to the later LTS releases Qt 5.12(.9 currently) or even 5.15(.0 currently), as Qt 5.9 is out of LTS support since end of May 2020. That may mean though, that we'd either need to drop support for macOS 10.10 and 10.11, or have separate builds for those still using Qt 5.9.

[Jojo-Schmitz]

For some bizarre reason the appveyor build did succeed! Is it the cache that needs to get updated/invalidated?
It apparently still uses the old QT:
Using Qt version 5.9.8 in C:/MuseScore/qt/msvc2017_64/lib

But does use the 'new' VS:
Generator is: Visual Studio 16 2019

Or has that Qt bundle been patched already? If so, this PR can get merged as is... into master and 3.x
(and update to Qt 5.9.9 would still preferably IMHO though)

[Jojo-Schmitz]

Ah, I see, I had forgotten to update build/appveyor/before_build.bat too

[Jojo-Schmitz]

Huh? It still succeeds? Strange...

Still:
Using Qt version 5.9.8 in C:/MuseScore/qt/msvc2017_64/lib
and also:
Generator is: Visual Studio 16 2019

[vpereverzev]

Regarding the Qt 5.9

Yeah, we're going to move on 5.12 LTS. The 5.15 is quite interesting, but it's so "fresh" for now, you know. So it should be safer to use 5.12 LTS

[Jojo-Schmitz]

Yes, Qt 5.12 should be the way to go for master / MuseScore 4 for now. Question is whether to maintain a Qt 5.9 version for macOS 10.10 and 10.11, or to finally loose those rather ancient OS versions

[shoogle]

macOS 10.13 High Sierra is the oldest version supported by Apple so I say drop them. Even 10.13 will be EOL by the time MuseScore 4 comes out, and possibly 10.14 as well. In any case, you will need recent Qt to support ARM / "Apple Silicon".

[Jojo-Schmitz]

Would the ARM processor/OS be supported by Qt 5.12(.9) already?

[shoogle]

There's been no official announcement yet about when they will release ARM binaries or which Qt version it will be for. They confirmed via Qt Mailing list that they will support macOS Big Sur, at least via Rosetta 2 x86 emulation, and they don't think there will be too much work to support ARM natively (they already support ARM on iOS and various Linux distros successfully compile Qt on ARM). Qt 5.12 goes EOL in December 2021, so it could arrive in 5.12.9 or they might decide to restrict it to newer versions.

[Jojo-Schmitz]

I don't think they'd retrofit it into the existing Qt 5.12.9, but maybe add it to ...10 or ...11