Video transcript feature not able to detect adding of cues in Safari

[bjester]

Observed behavior

The interactive video transcript does not load initially in Safari. The transcript feature overrides the addCue method on TextTrack's such that it can hook into the browser's native handling of text tracks and "listen" for when a cue has been parsed and added to the the track. Unfortunately, there isn't an event for this in the spec.

Expected behavior

The transcript and all the cues should load as soon as possible and shouldn't be dependent on the video playing or a cue becoming active.

User-facing consequences

Users of Safari have to toggle the transcript off and on to view it before playing the video. This may be frustrating for learners.

Errors and logs

No errors occur.

Steps to reproduce

1. Import a video with a first transcript cue that starts significantly after the video starts. For example, download "A is for Atom" from channel hifug-fagad
2. Open video in latest Safari
3. Enable the transcript if not already. The transcript should then show if it wasn't enabled.
4. Refresh
5. Observe transcript loading state until either a cue becomes active or the transcript is toggled.

Context

Kolibri 0.13 #5631
Observed in Browserstack
Observed in latest version of Safari