### What

* Part of #7353
* still missing hover preview, but this PR gets us to the minimum viable
product.

Generalizes the picking-rect mechanism to a large extent to include both
videos and error placeholders.
Picking code was in a quite poor state, so most of the changes here are
all about making it cleaner and easier to follow. Still some left to be
desired but had to stop somewhere ;-)

<img width="1264" alt="image"
src="https://github.com/user-attachments/assets/859aaa69-d367-4428-b7e7-df1e439fb7a9">


Overview over the refactor (it's almost commit by commit, but I went
back & forth on naming a bit and there's _some_ evolution that I didn't
clean up):
* replace `PickableImage` with a more versatile `PickableTexturedRect`
construct
* each visualizer now has a list of `PickableTexturedRect` to dump its
images into directly on the shared `SpatialViewVisualizerData`
   * similar to how ui label collecting works
* this allows all reasoning about pickable rects to be fully generic, no
need to know all visualizers
* the only exception are depth clouds which aren't images from a picking
perspective but get resolved to one
* pull the `PickingContext` creation out of the mighty `picking` method
* renames on `PickingContext` to make it more clear what's going on
* some more meaningful sub routines for the mighty `picking` method
    * it's still large and complex, but a lot better now 


### 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/7450?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/7450?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/7450)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
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