Add libass native decoder

[moi15moi]

This PR adds a new decoder for .ass subtitles to fix this issue: google/ExoPlayer#8435
It also partially fixes the following issue: #691

It is currently a draft because the following issues still need to be addressed:

• Currently, it only supports .ass subtitles muxed in MKV files. It would also needs to support "raw" .ass file.
• The way AssRenderer is added in SubtitleTranscodingTrackOutput.java and DefaultRenderersFactory.java is hacky.
• For frame-accurate rendering, we would need this issue to be resolved: Align subtitle output with video frame timestamps (overriding explicit subtitle timestamps) #2289
• draw_ass_rgba is too slow for 4K (3840x2160) bitmaps where the subtitle completely covers the screen. Neon optimization is needed (or maybe openmp?).

Note: We are no longer working on this PR. If anyone wants to use it as a base, feel free to improve it and open a non-draft PR.

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Why should Media3 provide a native decoder using libass?

It’s practically impossible to replicate libass behavior accurately.
The first commit that introduced SSA/ASS subtitle support in Media3 is this one, made in 2017 — that was 8 years ago, and Media3 still lacks support for many elements of the ASS/SSA spec.

Here's a screenshot showing the video with and without subtitles. Implementing this in SsaParser.java is far from trivial.

Also, here’s a video that contains a .ass subtitle (hardsubbed). The subtitle is the text: "CIBLE DE LA QUÊTE".
https://github.com/user-attachments/assets/af2b54b5-baf8-41d9-bc05-f97702757402
Again, implementing all the feature in java needed just to display the subtitle accuratly is basically impossible.

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Finally, note that this PR is the work of Raphael Dupuis, Sonny Uplavan, Vincent Zauhar and me.

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[icbaker]

I'm afraid we are unlikely to merge this PR as the maintenance burden is likely to be too high for the benefit it brings, as I previously mentioned in this comment: google/ExoPlayer#8435 (comment)

[Sparh4wk]

I'm afraid we are unlikely to merge this PR as the maintenance burden is likely to be too high for the benefit it brings, as I previously mentioned in this comment: google/ExoPlayer#8435 (comment)

well imho its for millions of ppl watching anime.. no one from exo community was able to do anything to support ass/ssa subs for more then 8 years, so I think benefits are too high for you to let this pass, when there is finally someone who is able to do it and spend his time to make it work... @moi15moi thanks for your hard work and dont let him discourage you.