Switch to fmp4 intead of mgepg

[zoriya]

Closes #534

Refs:

• https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/segment.c
• https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/hlsenc.c
• https://ffmpeg.org/ffmpeg-all.html
• https://www.agama.tv/demystifying-the-mp4-container-format/

Currently, this PR is not working, I guess timestamps or mp4 headers are invalid /shrug

[zoriya]

Maybe it would be wisest to switch to -f hls instead of -f segment to use their built-in fmp4 handling. Would need to check if it's possible to use segments from multiples transcode process this way tho.

[zoriya]

I'm realising supporting this would allow av1 streaming.

This would be huge to reduce bandwidth usage.

Definitely want to work on this on the following days

[zoriya]

Idea seem to work, need to:

• fix presentation times when restarting encode (see https://github.com/Dusk-Labs/nightfall/blob/master/src/profiles/video.rs for reference)
• fix keyframes count/duration for audio (used in segment computation)

[zoriya]

We would need to fix the assumption that keyframes are a file level things. This should move to a stream level thing.

It's legal to have audio/video with different segments. It's also legal to have two videos w/ different segments. The only constraint is that variant should have the same segments (aka video1/720p & video2/1080p needs same segments while video1/729p & video2/1080p can have different segments).

Doing this would also unlock video selection when the file contains multiple video streams.

[zoriya]

I'm thinking of first fixing this & data store in another PR before continuing this.