Fix empty edit timescale in sample tables

[padenot]

Empty edit durations use the movie timescale, but get_indice_table applied them directly as track ticks. Convert before computing the composition offset; the file in bug 2082877 should start video at 1/60 second rather than 16 seconds. This fixes the sample-table path left unchanged by #407.

Adds regression coverage for unequal timescales, negative offsets, large values and invalid conversions, and corrects existing sample-table expectations that encoded the same bug.

Validation: cargo test --workspace (219 passed, 1 ignored), cargo fmt --all -- --check, and git diff --check. The new regression failed before the fix (960 ticks instead of 1).

[Vanilagy]

Thanks for the swift fix!

Note that the segment_duration field is always in the movie timescale, regardless of if its empty or not, so the description might be a little misleading.

The two relevant fields, from ISO/IEC 14496-12:

segment_duration is an integer that specifies the duration of this edit segment in units of the timescale in the Movie Header Box.
media_time is an integer containing the starting time within the media of this edit segment (in media time scale units, in composition time). [...]

[padenot]

Sorry which description is a little misleading? Do we need to adjust a comment that I missed? Thanks for the report in any case.

[Vanilagy]

Oh, it was just a nit about the PR description: "Empty edit durations use the movie timescale" sounded like only the empty ones use the movie timescale, not all.