red_b->set_position (film, dcpomatic::DCPTime::from_seconds(10));
red_b->video->set_length (240);
sub_b->set_position (film, dcpomatic::DCPTime::from_seconds(10));
red_b->set_position (film, dcpomatic::DCPTime::from_seconds(10));
red_b->video->set_length (240);
sub_b->set_position (film, dcpomatic::DCPTime::from_seconds(10));