Audio Buffer Jack: Auto/2048/4096

[daschuer]

This fixes #11341

The issue was that the configured buffer was to small when selecting a big JACK buffer.
Unfortunately we receive the actual JACK buffer size via the Engine thread only, so we need to allocate the new buffer in the engine thread. But this is OK, because this will only happen during the very fist callback.

[JoergAtGithub]

LGTM

[daschuer]

Now the issue is solved. I have pipewire installed and can select various buffers while Mixxx is running with:
pw-metadata -n settings 0 clock.force-quantum 1024
for testing,

.. is now the default. And it starts counting underflows if you configure Jack to more than 1024 frames/period.
In this case you have to explicit adjust Mixxx/Portaudio for the selected buffer size.

[daschuer]

Done.

[JoergAtGithub]

Last CI run was Pass and only a typo in a comment was changed in last commit -> no reason to wait for CI to complete.
Thank you!