Imported audio ends before the film when slow-motion regions lengthen it #1372

Follow-up to #1355.

When a project has slow-motion speed regions, the exported film becomes longer than the imported audio clip as measured on the timeline ruler. Since #1355 the clip plays once at 1x over its own span, so the music stops before the film ends (measured on 23/10: about 2.5 s of silence at the end of a CLI export with slow-motion regions).

Expected: decide whether an imported audio clip that crosses speed regions should keep its 1x duration (current behaviour) or have its span extended so it still ends with the film.