sd_ass: assume negative durations are unknown durations, and handle them

The FFmpeg can officially not distinguish between unknown subtitle
durations, and subtitle durations being 0. (It documents the value 0
meaning unknown duration.)

In practice, at least the LRC demuxer signals unknown subtitle durations
with a negative value.

Assume negative durations mean unknown duration. Show subtitles with
unknown duration forever. Unless there's a subtitle event following it;
then reset the duration so that it ends on the new subtitle event.

Fixes #2244.

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