Support UMovieSceneSubTrack

[munkybutt]

Hey - I have a LS with sub-sequence track.
https://dev.epicgames.com/documentation/en-us/unreal-engine/cinematic-subscequences-track-in-unreal-engine

The export timeline action doesn't seem to find this and looking at the code it is because it is only searching for sub-shot track:

OpenTimelineIO-Unreal-Plugin/OpenTimelineIOUtilities/Content/Python/otio_unreal/level_sequence.py

Line 69 in cc1aac7

def get_shot_track(self):

As far as I can tell it they both behave pretty much the same, but the sub-sequence has an offset property to play the LS at in the world.

https://dev.epicgames.com/documentation/en-us/unreal-engine/API/Runtime/MovieScene/Tracks/UMovieSceneSubTrack

Are you open to a PR to add support for this?

[michdolan]

Hi, yes a PR to support this would be great, thanks!