sub: fix accidental subtitle overlaps

The fix_overlaps_and_gaps() function in dec_sub.c fixes small gaps or
overlaps between subtitle events. However, sometimes it could happen
that the corrected subtitle events could overlap by 1ms due to bad
rounding, making libass shift subtitles to reduce collisions. (The
second subtitle will be shown above the previous one, even if both
subtitles are visible only for 1ms.)

sd_ass.c rounds the timestamps when converting to integers for unknown
reasons. I think it would work fine without that rounding, but since
I have no clue why it rounds, and since it could be needed to ensure
correct timestamps with ASS subtitles demuxed from Matroska, I'd rather
not touch it. So the solution is to use already rounded timestamps to
calculate the new subtitle duration in fix_overlaps_and_gaps().

See github issue #182.