DVR: Support DVR as MP4 file.

[winlinvip]

MP4 is the foundation of #174 HTTP-MP4 and also the foundation of #299 MPEG-DASH.

TRANS_BY_GPT3

[winlinvip]

The brand isom is used by ffmpeg. We choose isom for DVR mp4.

[winlinvip]

As mp4 demuxer added to srs-librtmp, we also write a srs_ingest_mp4 to ingest mp4 file to rtmp stream. Because we only support live streaming, so we don't seek the mp4 file here and there, that means the mdat box should after moov, or mux in fmp4 brand.

FFMPEG always put mdat before moov, which make the loading very slow(must seek to tail of file):

ffmpeg -i source.200kbps.768x320.flv -c copy -y avatar.mp4

The srs_ingest_mp4 will seek moov when it's not non-seek mp4 file:

./objs/srs_ingest_mp4 -i avatar.mp4 -y rtmp://127.0.0.1/live/livestream

We can use the parameter to make sure moov at the beginning of file by -movflags faststart:

ffmpeg -i source.200kbps.768x320.flv -movflags faststart -c copy -y avatar_faststart.mp4

FFMPEG support fmp4 by -movflags frag_keyframe:

ffmpeg -i source.200kbps.768x320.flv -movflags frag_keyframe -c copy -y avatar_fragment.mp4