Fix video on web sometimes not showing last few frames for some videos

[Wumpf]

What

• part of Native H264 unable to decode last couple of frames of a video #8073

Tested on top of #8111 in Chrome @ Windows with a video that would previously not show the last two frames.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

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

[emilk]

This makes me think we should call flush in the Av1 decoder too (https://docs.rs/dav1d/latest/dav1d/struct.Decoder.html#method.flush)

[Wumpf]

Confirmed working on Firefox @ Windows as well.
... but I got really weird behavior from Firefox there: it refused to play the video at all, was still fine with Av1 though. Then restarted Firefox and it played the video poorly (with massive wait times and getting stuck). Then opened app.rerun.io to check if that regressed. That played perfectly with last two frames missing. Then I opened a new tab again with the version I'm testing and it played perfectly through to the end. 🙈🙈🙈