Obey audio timestamps if they don't deviate by more than some threshold.

Obey audio timestamps if they don't deviate by more than some threshold.

Previously we would ignore audio timestamps because they are not
contiguous in a sample-accurate way.

However with bugs like #1833 we do need to obey large discontinuities
in audio timestamps, otherwise we get large sync errors.

Here we change timestamp handling to ignore small discontinuities
in timestamps but not larger ones.