avformat: add SUP/PGS subtitle demuxer

Signed-off-by: Clément Bœsch <u@pkh.me>
avformat/srtdec: simpler and more lenient probing

Fixes Ticket #3935.
lavd/avfoundation: Use microseconds as common timebase.

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