avformat/hlsenc: program_date_time and append_list flags conflict

When program_date_time flag is present, in m3u8 playlist file each
segment has a corresponding EXT-X-PROGRAM-DATE-TIME value. The intial
program-date-time value is the actual current time at init and
each new segment increments this value by its duration. When append_list
flags is also present, existing playlist parsing by hls_append_segment
treats existing segments as new segments regarding the program-date-time
calculation. But it should not do that, because this way all real the
new segments' EXT-X-PROGRAM-DATE-TIME values will be shifted erroneously
by the sum duration of existing segments. Instead it should have
decremented the initial program-date-time value by its duration. This
would ensure that
the first new segment's program-date-time value had the current time as
it is expected.
This patch corrects this behaviour and prevent existing segments to
increment the value of initial_prog_date_time variable but decrements
it.

Reviewed-by: Steven Liu <lq@onvideo.cn>
Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>

libavformat/hlsenc.c

@@ -1187,6 +1187,7 @@ static int parse_playlist(AVFormatContext *s, const char *url, VariantStream *vs
                 is_segment = 0;
                 new_start_pos = avio_tell(vs->avf->pb);
                 vs->size = new_start_pos - vs->start_pos;
+                vs->initial_prog_date_time -= vs->duration; // this is a previously existing segment
                 ret = hls_append_segment(s, hls, vs, vs->duration, vs->start_pos, vs->size);
                 if (ret < 0)
                     goto fail;