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

#8117)

### What

* part of #8073

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


![image](https://github.com/user-attachments/assets/f804cc94-3870-46a9-a58d-6d4daf76f677)

### Checklist
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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/8117?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/8117?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
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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/8117)
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crates/store/re_video/src/decode/webcodecs.rs

@@ -178,6 +178,28 @@ impl AsyncDecoder for WebVideoDecoder {
 
         Ok(())
     }
+
+    /// Called after submitting the last chunk.
+    ///
+    /// Should flush all pending frames.
+    fn end_of_video(&mut self) -> Result<()> {
+        // This returns a promise that resolves once all pending messages have been processed.
+        // https://developer.mozilla.org/en-US/docs/Web/API/VideoDecoder/flush
+        //
+        // It has been observed that if we don't call this, it can happen that the last few frames are never decoded.
+        // Notably, MDN writes about flush methods in general here https://developer.mozilla.org/en-US/docs/Web/API/WebCodecs_API#processing_model
+        // """
+        // Methods named flush() can be used to wait for the completion of all work that was pending at the time flush() was called.
+        // However, it should generally only be called once all desired work is queued.
+        // It is not intended to force progress at regular intervals.
+        // Calling it unnecessarily will affect encoder quality and cause decoders to require the next input to be a key frame.
+        // """
+        // -> Nothing of this indicates that we _have_ to call it and rather discourages it,
+        // but it points out that it might be a good idea once "all desired work is queued".
+        let _ = self.decoder.flush();
+
+        Ok(())
+    }
 }
 
 fn init_video_decoder(