### What
Some small improvements in service of coming ffmpeg stuff

* Extracted from #7658
* Part of #7606

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7660?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7660?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7660)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

### What

* Most of #7607 
* Replaces #7658 

Tested this with videos containing b frames, both long and short.

<img width="1403" alt="image"
src="https://github.com/user-attachments/assets/12da809b-5aee-4c06-ba30-394d82073a2a">

(note how PTS != DTS, indicating that there's a b-frame _somewhere_ ;-))

Biggest risk overall with the approach taken is that we can't detect
dropped frames properly

Left to do before landing:
* [x] test 4k 60fps video material
* works smoothly in release, not at 60fps in debug builds on my mac
because of rgb->rgba conversions on our side (see todos below)
* [x] test black & white (yuv400)
* [x] works but my video test material gave me:
`re_video::decode::ffmpeg]
/\//Users/andreas/dev/rerun-io/internal-test-assets/video/greyscale_360p_h264.mp4,
codec: H.264 (avc1.64001E) decoder: [swscaler @ 0x3001d8000] [warning]
deprecated pixel format used, make sure you did set range correctly`
every time ffmpeg started, need to fix that to a log_once (if ffmpeg
says it's deprecated then I figure we may report that 🤷, just don't
spam)
* [x] test yuv444/rgb video
* worked, but the test 4.4s test video I used stops working starting at
3.2s, recovers fine though
* Direct play browser: Firefox says the file is corrupt, Safari refuses
to play. Chrome handles it
        * Rerun play browser: Same, but different errors
        * ... letting this issue slide for now 🤷 

What's left to do (beyond general video bugs I logged while working on
this) before calling this "done":
* download ffmpeg for the user or at least tell users what to do when
there's no ffmpeg (haven't checked even yet on the error message)
* add version & ability check for ffmpeg?
* consume 420p and friends directly to speed up processing
* try to provoke frame dropping and see if we can detect & handle that -
one of the shortcomings of the overall approach today is that we don't
track this!

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7962?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7962?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7962)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

---------

Co-authored-by: Emil Ernerfeldt <[email protected]>