tee muxer: produces incorrect start_time, PTS

[sbalko]

Summary of the bug:
The tee muxer changes the PTS times (and hence, the start_time). That is, when we transcode an input video (​https://drive.google.com/file/d/1qM40ZzOa10HJY7vZt0b-bx191eYBqqM_/view?usp=sharing, "Big Buck Bunny" WebM trailer) directly into MP4 like so:

ffmpeg -t 3 -i bunny.webm -vcodec libx264 -map 0:v bunny1.mp4

...we get an output video with a start_time property of 0.000000. In contrast, when using the "tee" muxer (for the sake of simplicity, with a single output only) like so:

ffmpeg -t 3 -i bunny.webm -vcodec libx264 -map 0:v -f tee "[f=mp4]bunny2.mp4"

...we get an output video, where the start_time property is 0.080000. We confirmed that both videos have the same number of frames, where the pkt_pts_time properties of bunny2.mp4 are +0.08 sec shifted.

How to reproduce:

ffmpeg version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 7 (Ubuntu 7.3.0-16ubuntu3)

We can also reproduce this issue with a more recent FFmpeg build:

ffmpeg version N-93945-g02333fe394 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 7 (Ubuntu 7.4.0-1ubuntu1~18.04)

[sbalko]

Big Buck Bunny trailer (WebM)

[sbalko]

...also attached the sample input video file to reproduce the issue.

[Cigaes]

I suspect the -avoid_negative_ts option us responsible here. Does setting it to 1 for MP4 produce the same effect as tee? And setting to 0 for tee produce the same effect as MP4?