Avdecode/avfoundation: use AudioConverter & fix thread concurrency

[toots]

This PR adds one feature and fixes one issue:

New feature

This PR adds support for PCM format conversion using AudioConverter. This extends audio support to unpacked inputs and also cleans up conversion from non-interleaved to interleaved format.

This addresses this ticket: https://trac.ffmpeg.org/ticket/9502

Fix thread concurrency

The thread model in the avfoundation runs two threads concurrently, one that pulls data out of the input devices and one that pulls data into the FFmpeg stack. There was a lock preventing concurrent access to captured data preventing concurrent access but this was missing a condition, resulting in dropped frames. This PR adds a it as well.

[retokromer]

Please consider #153

[BtbN]

Patches should be submitted to the ffmpeg-devel mailing list using git format-patch or git send-email. Github pull requests should be avoided because they are not part of our review process and will be ignored.

See https://ffmpeg.org/developer.html#Contributing for more information.