Extended subtitles support across containers

[wiedymi]

Summary

This PR adds end-to-end subtitle support across containers, parsing/muxing, and conversion workflows.

What’s included

• Core subtitle track support for MP4/MOV/MKV pipelines.
• Subtitle parsing/formatting utilities for:
  • webvtt
  • srt
  • ass / ssa
  • tx3g
  • ttml
• ISOBMFF subtitle support expanded with tx3g and ttml mapping/handling.
• Conversion API support for subtitle tracks via ConversionSubtitleOptions:
  • discard
  • target codec
  • track-specific callback support (same pattern as video/audio options)
• ASS/MKV robustness fixes for real-world edge cases, including ReadOrder-related reconstruction.

Docs and examples

• Added/updated subtitle extraction and muxing examples.
• Updated conversion guide and supported formats/codec documentation.

Tests

• Added subtitle-focused test coverage including:
  • ISOBMFF subtitle tests
  • Matroska subtitle tests
  • Subtitle parsing tests
  • Subtitle extraction tests
  • Subtitle conversion tests
  • Advanced ASS/SSA handling tests

Commit structure

This branch is intentionally split into focused commits:

1. Core subtitle support (MP4/MOV/MKV).
2. tx3g/ttml support for MP4/MOV.
3. Conversion API subtitle options.
4. Conversion test coverage.
5. ASS/MKV parsing hardening.

Notes

• This PR includes non-trivial subtitle fixture updates under test/public/subtitles/.
• No public API removals; this extends subtitle capabilities.

[Vanilagy]

Thanks! This is a large PR and thus will take some energy to review, so given that this feature is not on the immediate roadmap (although it is on the roadmap!), I'll deal with this PR at a later date. But don't worry, I won't forget about it!

[wiedymi]

Take your time

Thanks! This is a large PR and thus will take some energy to review, so given that this feature is not on the immediate roadmap (although it is on the roadmap!), I'll deal with this PR at a later date. But don't worry, I won't forget about it!

[wiedymi]

@Vanilagy this one is rebased as well

[kzahel]

Hey I'm just leaving my support here I started using media bunny and a project and think it would be really cool to have subtitle support

[wiedymi]

@Vanilagy let me know when you will have time to revise the PR to your vision and finally ship it 👀