Support containerless formats

[joeyparrish]

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

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

Some HLS content uses containerless formats, such as raw AAC or raw MP3. In these formats, there is no timestamp information. This not only complicates the building of the segment index in the HLS parser, but it requires special timestamp management at the MediaSource level because these formats implicitly force their SourceBuffer into sequence mode.

Without special handling, the timestamps are never right after seeking, even if we synthesize correct SegmentReference timestamps during the playlist parsing.

Describe the solution you'd like

We should skip these formats in the HLS parser's timestamp discovery stage. We should also mark these streams as being containerless. Finally, StreamingEngine should use this containerless flag to manage timestamps at the MediaSource level (set timestampOffset every time the buffer is cleared, and include the first appended segment reference's start time).

Describe alternatives you've considered

We could reject containerless formats, but I'd prefer to improve our support for them for the sake of legacy HLS content.

Additional context

Related to #1083 (raw AAC).
Sequence mode docs: https://developer.mozilla.org/en-US/docs/Web/API/SourceBuffer/mode
Spec text around sequence mode: https://www.w3.org/TR/media-source/#dom-appendmode-sequence

[avelad]

@ismena , can you consider this issue for your next roadmap planning? (github label) There is a lot of legacy content to support and this would be very useful.

[ismena]

Yes!