Add support for WebVTT subtitles embedded in webm files

[ale5000-git]

Sample file: https://gofile.io/d/2jbaFN
This fork of LAVFilters support it:
https://github.com/clsid2/LAVFilters

It is included in K-Lite Codec Pack 16.0.9.

Ref: #18

Could you please add support for WebVTT subtitles embedded in webm files?

[pinterf]

Well, O.K., I made it to recognize the WebVTT format but no formatting is supported. (and as I see neither does it MPC-HC with the above sample)
It would be nice that as a minimum formatter tags must to be omitted. Leaving the bare text.
Info for myself: https://w3c.github.io/webvtt/#webvtt-cue-class-span
Writing a parser for that is not a two minutes task - at least not for me :)

[clsid2]

MPC-HC converts a few tags to SSA format, and strips a few unsupported ones.

For example see last few commits from today:
https://github.com/clsid2/mpc-hc/commits/develop

[pinterf]

Great, thank you clsid2, it's a hot topic, I'll then wait a bit.

[ale5000-git]

@pinterf: The latest MPC-HC plus this pull request already has a considerable support.

[pinterf]

Yep, the earlier changes were already integrated, still waiting a bit to settle down.

[ale5000-git]

Can this be considered done or there is something missing?

[pinterf]

I think it is considered done, except that I haven't checked since March whether it was developed further in MPC-HC.