refactor(soundsourceffmpeg): Fix compatibility with FFmpeg 5.1 [WIP]

[Holzhaus]

Fixes #10856.

TODO:

• Use ifdefs to maintain compatibility with older ffmpeg versions
• Find bug that causes two tests to segfault:

The following tests FAILED:
	697 - SoundSourceProxyTest.seekForwardBackward (SEGFAULT)
	699 - SoundSourceProxyTest.seekBoundaries (SEGFAULT)

If someone wants to take over, feel free to do so. I don't know when I'll find the time to continue working on this.

[uklotzde]

Thanks, Jan. I have already re-added the deleted code behind version flags. Would have been easier to do this before rewriting/deleting. No time to test yet.

[uklotzde]

Unfortunately, the Mixxx build fails when using the custom FFmpeg static libs due to undefined symbols in libavcodec. Worked for 5.0. Might try later, no prio.