Fix corrupted audio and video at playback start in ogg containers.
When demuxing ogg containers, FFmpeg will seek to the first packet in the
chosen stream that matches the seek criteria. However, it does not advance
or rewind other streams in the container to match this timeline.
It's an open question whether FFmpeg is doing the wrong thing here:
To keep things working in the meantime negative audio start times will
be clipped to zero when being considered for seeking.
- Fixes non-audio timestamps being incorrectly offset when negative
timestamps are discarded from the audio stream.
TEST=new unittest, extended existing tests.