New 2d heuristic -- split any bucket with more than 1 image

[jleibs]

What

• Resolves: Logging images to different child-of-root entity paths should result in separate SpaceViews #5145

I decided that if images are nested then we should still stack them when bucketed.
However, if multiple images are logged as siblings (or other non-nested hierarchies), they should get their own reviews.

The basic logic is as follows:

• Bucket the images.
• If the root of the bucket is an image (i.e. the contents are nested image) then use it.
• Otherwise, check to see if there is more than 1 non-nested image or non-nested depth-image. (We allow multiple segmentation images to layer). If there aren't we return it.
• If yes, then split the bucket and recurse.

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 newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• 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!

Extra checklist

• Reset viewer and test all nightly examples

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

The code confuses me, but if it makes all the test go green I don't want it holding up the release.

[emilk]

running cargo rerun reset && cargo rerun and clicking on Objectron sometimes leads to this pretty broken state:

Notice that the top right viewport has a huge 2D image in a 3D viewport

Hitting "reset viewport" fixes it.

• Investigate first-frame heuristics for our default examples #5156

[emilk]

I suggest we remove objectron from the default example list.