Staging Belt & use in PointCloudBuilder

[Wumpf]

TODO:

• fix horrible debug performance
• size point cloud builder correctly / let it handle chunks
• put all writes for point cloud onto the staging belt (as a proof of concept! other things can wait)
• improve staging belt ergonomics further
• do we REALLY need to take a dependency on ouroboros?

Fixes #592
First half of #426

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I've added a line to CHANGELOG.md (if this is a big enough change to warrant it)

[Wumpf]

haven't tried it, but gfx-rs/wgpu#3307 might help with debug perf