discard_pid can be quite expensive, so let's cache it and recalculate it on
every packet start.

ffmpeg -y -i samples/MPEG-VOB/sdtv/RAI.ts -c copy -map 0:v:0 -map 0:a:0 -f mpegts /dev/null

Before:
   1685 decicycles in handle_packet,  523483 runs,    805 skips

After:
    883 decicycles in handle_packet,  523505 runs,    783 skips
Signed-off-by: Marton Balint <cus@passwd.hu>