avformat/hlsenc: compute video_keyframe_size after write keyframe

avformat/hlsenc: compute video_keyframe_size after write keyframe

fix ticket: 8636
When write keyframe and the keyframe is the frist packet of the segment,
then compute the size of the keyframe which have been write into segment
first packet. and set the start position of the segment, should not use
avio_tell(vs->out) to get the keyframe position, because it can be set
to 0 if close at above of the workflow, that maybe inaccurate, but the
start_pos can be used here, because start_pos is set after write
the previous packet.

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: liuqi05 <liuqi05@kuaishou.com>