Rework RAOP audio sync

commit 9ec7421138adef73ef9cb83b74e046092fecdd6a that fixed audio timestamp calculations revealed an issue in the way RAOP was calculating and setting time syncing.
Rework how things are done: We now calculate the audio card latency and simply drop the initial audio frames amounting for such length of time. From that point on, when we get too far behind we drop all queued audio and restart.

Rather than dropping a whole audio packet at a time (352 frames) we now drop frame by frame, so re-syncs are much smoother.

Surprisingly, this simpler method gives much better results. Synchronisation across multiple airplay device is almost perfect (tested across 2 macs and one linux). Remote audio and video sync are spot on.

Fixes #11530