It's the minimum of all child protocols max_packet_size. Can be used
like this:

ffmpeg -re -i cctv.mp4 -c copy -f mpegts \
  -protocol_whitelist 'tee,file,udp' \
  'tee:out.ts|udp://127.0.0.1:6666?pkt_size=1316'
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>