The audio output might take too long to initialize the master clock

cf. #26825 (closed)

Depending on the audio device (specially with BT ones), video might get glitchy (one or few frame drops) when starting.

Here is the reason: at the beginning of playback:

• The input pushes a lot of data at the beginning of the playback
• But the audio time_get might return an error since the audio buffer has not started yet.
• Therefore, it is not possible to have a valid master audio clock during that first input burst.

The problem is that the next data burst might come later (after more than 1 seconds), resulting on the audio master to not be updated.

The only valid solution I see is to call the audio output time_get callback when a slave clock is trying to convert its pts, to update the master clock offset ASAP.

This strategy could be triggered only:

• when starting the playback
• and if the audio clock has not been initialized.