This is a refinement of the fix introduced in 3c1a9cd7c4.
Further investigation of the issue reveals that the cause of the bad
timestamps which result in error messages or hang is when the `apad`
filter does not receive any input audio frames. This happens if the
seekpoint is after the end of audio. The existing seekpoint checks can't
cover this, because in a deskshare file the video stream can continue on
after the audio stream ends.
The `,asetpts=N` filter was added to deal with this problem, and it
works in most cases. But there is a case where it fails - when the
"mismatched length" audio stretching to work around broken recordings
from BigBlueButton 0.81/0.90 kicks in.
The issue there is that the `atrim=start=S` filter (used due to the
difficulty of calculating seeks when stretching) hits the invalid
timestamps and hangs.
I'm working around this issue with a "defense in depth" combination of
two changes:
* Move the `,asetpts=N` filter to be applied before the audio stretch
filters. This fixes the processing hang.
* Adjust the conditions on the "mismatched length" audio stretching so
that it only gets applied on audio files likely to be from extremely
old BigBlueButton versions - those with audio in wav files, or encoded
to vorbis.
BigBlueButton has been using audio recorded directly to opus by
FreeSWITCH for quite a while, and the handling for gaps or lost packets
is done in current BBB versions by a combination of the libopus decoder
and the use of ffmpeg's `aresample=async=1000` filter to dynamically
stretch, squish, or fill in audio so it becomes a continuous stream
that's locked to the file timestamps. Applying the "mismatched length"
processing on top of that is probably making audio sync issues worse.
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