NOTE: About pkt_size=1200
WebRTC must need pkt_size=1200
If pkt_size > 1200 (most tool > 1200, for example: ffmpeg default 1472), we need to de-payload after re-payload
But now, We support re-size pkt_size in VP8 and VP9, You can use any pkt_size value in VP8 and VP9