Add wait_keyframe feature in the record module

[maxdolle]

I'm using the record module with a low record_interval (1 minute).
When I try to merge the segment files, the video stutters because the segment files have no video frame before the first key frame, and the first key frame can only appear after a few seconds.

This pull request provides an option "record_wait_keyframe"; when enabled, the next segment file always starts with a keyframe.

Tested with nginx 1.13.8, and behaviour verified with ffprobe -show_frames.

First video frame in a segment file with record_wait_keyframe disabled:

[FRAME]
media_type=video
stream_index=1
key_frame=1
pkt_pts=1251
pkt_pts_time=1.251000
...
[/FRAME]

First video frame in a segment file with record_wait_keyframe enabled:

[FRAME]
media_type=video
stream_index=1
key_frame=1
pkt_pts=80
pkt_pts_time=0.080000
...
[/FRAME]

[baxerus]

You have the code written already, but when you using such short recording intervals you could look into using the HLS segmenting feature instead (with long segments). The segments are well formed in terms of starting with keyframes.

[maxdolle]

That's a good idea, it would make the server compatible with audio-only streams too.
I haven't managed to make the fragment size higher than 10s though, I'm get #EXT-X-DISCONTINUITY.

[baxerus]

This a bug that is closed even it is not fixed ☹️.
See: #152
As a workaround you simply also have to set hls_max_fragment to something a GOP-size or two longer that hls_fragment, because it is calculated wrong.

Perhaps you could also file a new bug report for this.

[sergey-dryabzhinsky]

@maxdolle I like idea of new option - the same meaning as for live secton option

[sergey-dryabzhinsky]

Please, add definition to doc/directives.md