fix bug: subtitle no longer blink in low latency

[Florent-Bouisset]

The bug

When playing a video in low-latency mode with subtitle, the subtitle were disappearing and reappear very quickly making the subtitles "blink".
This was only experienced in low-latency mode.

---

Explanations

It was due to the size of the subtitles segments that differs in normal mode vs low-latency:

• Normal mode: around 2 seconds
• Low latency: around 2 seconds with chunks of 40 milliseconds

When sorting segments in the buffer, there is a logic that compare which segments should come first, and it determines if there are segments that are the "same" or if they overlap.

The logic that compute if two segments are the "same" were relying on a constant MAX_DELTA_BUFFER_TIME equals to 200ms.
200ms of tolerance is fine for segments that lasts 2s, but for segments of 40ms in low-latency it's way too high, and segments of 40ms that were different were considered "same".

As a correction, the tolerance is variable and depends on the segment duration that is being inserted. It's equal to one fifth of the duration of the segment.

---

Additionally, there was another issue on parsing of the cues:
the end: 278946541.9999333 of the cuesGroup has more precision than the end: 278946542.039 of the cuesElement.
In very specific case, there can be time < cuesGroup.end and time > cuesElement.end.
In such case the cuesElement is not displayed.
Again adding a small tolerance fix the issue

{
    "start": 278946541.9999333,
    "end": 278946542.0399333,
    "cues": [
        {
            "start": 278946541.999,
            "end": 278946542.039,
            "element": {}
        }
    ]
}

[peaBerberian]

Nice!

Small nitpick on the PR message as it wasn't limpid:

It was due to the size of the subtitles segments that differs in normal mode vs low-latency

To note that this is not inherent to DASH low-latency. Packagers could theoretically have smaller or bigger segments/subsegments where the issue could be much rarer or not exist.

[peaBerberian]

Could have a small JSDoc to describe its purpose once we forget the issue or somebody else look at it

[peaBerberian]

Your comments are weirdly sometimes (not the first time) strangely indented more than the code it comments. Is it on purpose?

On another subject, we could explain here that this allows to avoid having a very small gap without subtitles when that was probably not wanted.

[peaBerberian]

Nice, it seems that the linter is angry though. You can check it locally by doing an npm run lint?

[peaBerberian]

IMO we should still be explicit about the length check here. I don't know why the linter isn't failing here.

[Florent-Bouisset]

I have changed it, linter was fine with both ways

[peaBerberian]

LGTM, waiting for the CI and then if it goes well, merging it