Attempt to provide an FFmpeg/libav alternative for audio postprocessing

[lminiero]

This pull request tries to provide an alternative to libogg in the post processing utility. Specifically, it allows you to make use of FFmpeg/libav for baking the .opus file, pretty much as we already do with video for the .webm file.

To use this, pass the JANUS_PPREC_NOLIBOGG=1 env variable:

JANUS_PPREC_NOLIBOGG=1 /path/to/janus-pp-rec /path/to/file.mjr /path/to/file.opus

It probably doesn't work as expected right now, so feedback is welcome before integrating this in the code base.

[jing3018]

@lminiero I have two issues with the audio postprocessing
1.I cann't get the correct .opus file with the JANUS_PPREC_NOLIBOGG=1

[root@iZ237deuahcZ video_test]# JANUS_PPREC_NOLIBOGG=1 ./janus-pp-rec 1.audio.mjr 1.1.opus
1.audio.mjr --> 1.1.opus
File is 3782910 bytes
Pre-parsing file to generate ordered index...
This is an audio recording, assuming Opus
Counted 28792 RTP packets
Counted 28792 frame packets
Using libogg: false
1.1.opus is 1812126 bytes
Bye!

[root@iZ237deuahcZ video_test]# ffprobe 1.1.opus
[ogg @ 0x17972e0] Codec not found
1.1.opus: End of file

[root@iZ237deuahcZ video_test]# opusdec 1.1.opus 1.1.wav
Decoding complete.
This doesn't look like a Opus file

2.audio postprocessing duration not correct , same issue in master branch

[root@iZ237deuahcZ video_test]# ./janus-pp-rec 1.audio.mjr 1.opus
1.audio.mjr --> 1.opus
File is 3782910 bytes
Pre-parsing file to generate ordered index...
This is an audio recording, assuming Opus
Counted 28792 RTP packets
Counted 28792 frame packets
Using libogg: true
Writing .opus file header
ts begin=[3208303045] ts end=[3298892485]
duration=1887280000
1.opus is 1818830 bytes
Bye!

[root@iZ237deuahcZ video_test]# ffprobe 1.opus
Input #0, ogg, from '1.opus':
  Duration: 00:31:22.68, start: 0.000000, bitrate: 7 kb/s
    Stream #0:0: Audio: opus, 48000 Hz, stereo, fltp

from rtp timestamp, I get the audio duration is 1887.28 seconds
from .opus file , I get the audio duration is 31*60 + 22.68 = 1882.68, it lost 5 seconds 。

[lminiero]

@jing3018 yes, I know FFmpeg based conversion is broken: it was an attempt (failed) at having an alternative to libogg for Opus processing. I'll close this pull request as it's definitely useless.

About the other problem you found out, please open a new issue, as it may be related to the problem we were trying to fix in the first place (that is, occasionally progressively desynced audio and video).