Seamless fallback

[philippotto]

This PR rewrites the logic for fallback rendering. In short:

• the shader is now able to render arbitrary zoom steps. the lookupbuffer decodes which where the available bucket is and which zoomstep it has (instead of having a look up buffer for each zoom step)
• the ortho bucket picker is able to select buckets for multiple zoom steps
• the arbitrary bucket pickers only select one fallback at the moment. I think this is fine, since poorer quality is barely usable for flight mode tracing.
• the loading strategy can be selected within the dataset settings.
• also, forgive me father for I have sinned: I made some style tweaks to the settings sidebar, because I felt a strong urge to do so 🙈

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• I tested both loading strategies while throttling the internet connection to dsl 6000

Issues:

• fixes Seamless Fallback Rendering #3805

Todo:

• have lab test it
• find better fix for flight mode bucket picking inaccuracy
• write migration to add best-first loadingStrategy as preferred setting for all existing task types

---

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Needs datastore update after deployment
• Ready for review

[daniel-wer]

Great PR, thanks for all the effort in improving the code quality and flexibility of the GPU rendering!

I'll have a look at whether it's easy to migrate the existing recommendedConfigurations.
Will test now :)

[daniel-wer]

Worked really well during testing for me, I didn't experience any black spots 👍

I did notice a lower frame rate during flight mode tracing (compared to orthogonal) - ~15FPS compared to ~30FPS and it felt kinda laggy, but I can't say how that compares to the status quo. Let's see what our testers say :)