Use vcpkg for MinGW dependencies

[messmerd]

This PR finally switches most of the MinGW build's dependencies from tobydox's PPA to vcpkg.

Since the tobydox PPA was created for Ubuntu 20.04 (focal), its dependencies were several years out-of-date. But now with vcpkg, we finally have the latest versions of most of our dependencies.

There are some exceptions however:

• Qt [crashed GitHub's build runner last time I tried building it due to memory usage]
• libgig [not supported by vcpkg on MinGW yet]

For these exceptions, I'm continuing to use the tobydox PPA for now, but in a follow-up PR, I'd like to fully remove our dependence on the PPA.

Also, suil is neither available on the tobydox PPA nor vcpkg yet, so we will need to add that to vcpkg in the future.

This PR adds the following to MinGW builds

• mp3/opus support
• per-note panning in Sf2 Player
• DirectSound and WASAPI audio backends in PortAudio
• LV2 plugin support (theoretically)

Closes #6871

[qnebra]

Qt [crashed GitHub's build runner last time I tried building it due to memory usage]

Github have hard limits of how much memory can be used in build runners. Looks like building QT can really easily go above its 10 GB limit. Maybe issue is here.

[messmerd]

I tested this on Windows and found no issues.

The mp3 support works and SF2 Player has per-note panning now. I haven't tested LV2 plugins, so it's possible that may require a follow-up PR.

This PR is ready to merge if I can get an approval.