[2.3.4] Fix compatibility with FFmpeg 5.1

[uklotzde]

Needed to keep Mixxx working in Fedora 37:

• Based on and a replacement for refactor(soundsourceffmpeg): Fix compatibility with FFmpeg 5.1 [WIP] #10857
• Minimum viable changes to fix the build errors while ensuring backwards compatibility and not segfaulting at runtime

If someone wants to improve the awkward and brittle C-style memory management of AVChannelLayout using RAII please do so later. I won't.

[uklotzde]

@Holzhaus Please verify.

[Holzhaus]

Thanks, works like a charm. For building with -DWARNINGS_FATAL=ON, I also had to incorporate this unrelated fix: #10863