SSA spec allows the lines in any order, so they must all have an end time: http://moodub.free.fr/video/ass-specs.doc The Matroska write-up of SubRip assumes the end time is present: https://matroska.org/technical/specs/subtitles/srt.html This will massively simplify merging issue:#6595 PiperOrigin-RevId: 279926730
