FFmpeg: Handle FFmpeg extradata in dedicated class

[neo1973]

Description

This ensures that everything is allocated with av_malloc and freed with av_free. The padding is also handled consistently. And as a bonus it's more ergonomic than handling the raw data.

Things to consider for reviewers:

• Is it a good idea to let FFmpegExtraData throw if allocation fails?

Motivation and context

Mixing the allocation and deallocation methods is very bad, see #23080.

How has this been tested?

Only very light runtime testing so far!

What is the effect on users?

No crashes.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[lrusak]

Nice change! It really improves the interface and readability.

[fritsch]

Please rebase. I hope I can review it during weekend as well. It's a very good approach architecture wise, so I see this as major improvement.

[fritsch]

Thanks much! Let's find out if we hit some edge cases.