Fix various bugs in subtitle/ccap verification.

Fix various bugs in subtitle/ccap verification.

Check that subtitles don't overlap reel boundaries, and fix a few
tests that trip this check.

Fix confusion when calculating subtitle timings during verification
where the picture asset frame rate was being used rather than the
subtitle asset's edit rate.

Do the subtitle timing verification for Interop as well as SMPTE
subtitles.

Take <StartTime> tags into account when checking subtitles, even
though Bv2.1 says they should be set to 0.

Rename Time::as_editable_units to Time::as_editable_units_ceil
and add a _floor variant, then use that to round down when checking
reel boundary overlaps.