avsb: fast start with airplay devices
On top of !5873 (merged) and !5876 (merged). Only the last commit should be reviewed.
the setRate atHostTime function can't really be trusted or was used incorrectly. Start at the given date manually using the whenDataReady callback.
This fixes the following:
- Immediate playback with airplay devices (tested with Homepod) on iOS/tvOS (don't have to wait 2 seconds to hear a sound anymore).
- Working playback on macOS without the necessity to manually increase the pts-delay, but the audio need 2 seconds to start (A/V sync OK, but video paused for 2 seconds).