Allow for custom visualizer with custom shader that integrates into existing view

[Wumpf]

Related

• Extensibility tracking issue #3087
  • puts "Modify Views" to "level 1.8" - depends a bit on the view how much can be extended since views have a bit different approaches to how they draw things

What

Previously, it was already possible to add fully custom views to the Viewer. However, it was pretty much impossible to add a visualizer to an existing View, e.g. to add a new kind of visualization to the 3D view. This PR enables exactly that!

Most of this PR is an example demonstrating such a custom visualizer extending the 3D view with a new visualization. However, there's plenty of fixes and function/struct/trait exports that had to happen to make this possible!

DRAFT TODO:

• split out general improvements that also apply to custom views into a separate PR?
• visualize something interesting
• make custom visualizer pickable
• give custom visualizer outlines
• write readme for the new example
• add to viewer customization doc page (can be short for starters)

Known limitations:

• this is mostly useful for 2D, 3D and Map Views: on other view types, custom visualizers won't be able to affect what is shown since this is all happening in the view's ui method
  • can we make all views at least overlay with re_renderer draw data like we do on the map view?
• most of the newly exposed interfaces are not considered stable
• there may be still functionality of re_renderer that isn't exposed that should be

[github-actions]

Web viewer built successfully. If applicable, you should also test it:

• I have tested the web viewer

Result	Commit	Link	Manifest
✅	f188c23	https://rerun.io/viewer/pr/9820	+nightly +main

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[github-actions]

Latest documentation preview deployed successfully.

Result	Commit	Link
✅	f188c23	https://landing-be3u9gnsy-rerun.vercel.app/docs

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