avformat/mpegts: print packet size warning only if new size differs from old

No case is known to have triggered this, but its more correct to check that the
new size differs.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mpegts: fix pos47_full

The value was off by a packet

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mpegts_get_pcr: dont loose a packet when resyncing

This matches how read_packet() works

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegts: resync from the smallest packet size on

This avoids the loss of a packet in many cases if the packet size was wrong.
It also improves detection of packet size changes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>