Conversion to Tx3g subtitles generate wrong duration

[Marc Saurfelt]

Subler version: 1.6.3 (existed before I think)

Subtitles are correctly OCR’ed, but their duration is wrong (they last until the next subtitle)

See https://bitbucket.org/galad87/subler/issues/577/srt-export-always-starts-subtitles-timing for details of my workflow (and an example of exported SRT file).

I have verified that the problem is not in the export (the .mp4 file generated by Subler has the same issue)