Speed up data density graph by rendering them more coarsly

[emilk]

What

• Closes Frame drop when dragging data in Streams view to either side #7223

We would render individual points a bit too often before. Even though it would only take 0.5ms on a laptop, there could be a lot of density graphs on the screen at once, and the milliseconds would add up quickly - especially on Wasm, where everything is slower.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
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• 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

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