DVD PTS discontinuities are now handled by 'flattening' the timecodes of incoming packets. This prevents AVFormatDecoder getting stuck buffering video frames when the timecodes jump backwards.

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>
VTS title and clean up menu looping detection.

Querying the number of buttons as soon as a subpicture stream change
notification appears is generally not possible as the button
information is carried in NAV packets, which probably haven't been
read yet.

Fixes #11445

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>