One that is fine enough to represent all DV audio sample rates. Audio
packet durations are now sample-accurate.

This largely undoes commit 76fbb005. To
avoid breaking the issue fixed by that commit, resync audio timestamps
against video if they get more than one frame apart. The sample from
issue #8762 still works correctly after this commit.

Slightly changes the results of the lavf-dv seektest, due to the audio
timebase being more granular.