avformat/mpegts: add scan_all_pmts option

This allows selecting if the demuxer should consider all streams to be
found after the first PMT and add further streams during decoding or if it rather
should scan all that are within the analyze-duration and other limits

Fixes Ticket3762

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegts: improve first valid PMT heuristic

This checks for audio+video instead of streams > 2
Fixes Ticket4090

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