Support addThumbnailsTrack in SRC_EQUALS

[absidue]

Have you read the FAQ and checked for duplicate open issues?
Yes

Is your feature request related to a problem? Please describe.

Since version 4.4.0 (#5502) the UI will display thumbnails when hovering over the seek bar when they are provided. Unfortunately this is currently only usable when the load mode is MEDIA_SOURCE, as Player.addThumbnailsTrack() throws CANNOT_ADD_EXTERNAL_THUMBNAILS_TO_SRC_EQUALS when you call it when the load mode is SRC_EQUALS. While that error is correct, it also makes it impossible to use the thumbnail display feature with SRC_EQUALS.

Describe the solution you'd like

A way to pass thumbnail tracks to the UI even when the load mode is SRC_EQUALS. As shaka-player can handle them completely fine when the load mode is MEDIA_SOURCE, it means that most of the necessary infrastructure already exists, it just has to be adapted for use with the load mode being SRC_EQUALS.

Describe alternatives you've considered

Building a custom seek bar component with custom thumbnail handling and registering that with the UI.

[theodab]

Sure, that'd be possible. The thumbnails feature as it works today relies on the existence of a manifest, but there's no reason we couldn't add some alternate container for storing user-added image tracks that we could use while loaded in src-equals mode.

[avelad]

I'll work on it in a few weeks.

[avelad]

I'll work on this after it's merged #5752

[avelad]

@absidue I created #5778

[absidue]

Thank you! :)