When audio_preload option is set output media stream a/v interleaving is
broken because of "wrong sign issue" in interleave_compare_dts.
I'm willing to provide patch solving this issue and also enabling audio
stream shift in both directions (currently audio_preload is 0-int_max).
Probably audio_preload can be renamed to something like audio_shift? (it
seems no one uses audio_preload option because of this issue)