avformat/mpegtsenc: fix incorrect PCR selection with multiple programs

avformat/mpegtsenc: fix incorrect PCR selection with multiple programs

The MPEG-TS muxer had a serious bug related to the use of multiple programs:
in that case, the PCR pid selection was incomplete for all services except one.
This patch solves this problem and selects a stream to become PCR for each
service, preferably the video stream.

This patch also moves pcr calculation attributes to MpegTSWriteStream from
MpegTSService. PCR is a per-stream and not per-service thing, so it was
misleading to refer to it as something that is per-service.

Also remove *service from MpegTSWriteStream because a stream can belong to
multiple services so it was misleading to select one for each stream.

You can check the result with this example command:

./ffmpeg -loglevel verbose -y -f lavfi -i \
  "testsrc=s=64x64:d=10,split=2[out0][tmp1];[tmp1]vflip[out1];sine=d=10,asetnsamples=1152[out2]" \
  -flags +bitexact -fflags +bitexact -sws_flags +accurate_rnd+bitexact  \
  -codec:v libx264 -codec:a mp2 -pix_fmt yuv420p \
  -map '0:v:0' \
  -map '0:v:1' \
  -map '0:a:0'  \
  -program st=0:st=2 -program st=1:st=2 -program st=2 -program st=0 -f mpegts out.ts

You should now see this:

[mpegts @ 0x37505c0] service 1 using PCR in pid=256
[mpegts @ 0x37505c0] service 2 using PCR in pid=257
[mpegts @ 0x37505c0] service 3 using PCR in pid=258
[mpegts @ 0x37505c0] service 4 using PCR in pid=256

Fixes ticket #8039.

v2: a video is stream is preferred if there are no programs, just like before
the patch.

Signed-off-by: Marton Balint <cus@passwd.hu>