getParallelAtTime method
Returns the SmilParallel corresponding to currentTime.
Returns null if the section has no audio version.
Implementation
SmilParallel? getParallelAtTime(Duration currentTime) {
if (!hasAudio) return null;
assert(currentTime <= audioDuration!);
final parallel = smilParallels.firstWhere(
(par) => currentTime >= par.clipBegin && currentTime <= par.clipEnd,
);
return parallel;
}