[Draft/deferred] Cuckoo hashing for data rendering

[philippotto]

This branch was created in March 2021 and deprioritzed shortly after. I opened the PR for it to avoid it being completely "gone".

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• abc

Issues:

• fixes #3915

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(Please delete unneeded items, merge only when none are left open)

• [ ] Updated (unreleased) changelog
• [ ] Updated (unreleased) migration guide if applicable
• [ ] Updated documentation if applicable
• [ ] Adapted wk-connect if datastore API changes
• [ ] Adapted wk-libs python client if relevant API parts change
• [ ] Needs datastore update after deployment
• [ ] Ready for review

[philippotto]

Short update:

• cuckoo hashing is implemented (in JS) and should work
• the integration of the new hashing is blocked because the murmur hash is not compatible with webgl 1
• switching to webgl 2 is prepared, but does not work for some reason with the look up buffer (which is a 2-channel float texture)
• I updated threejs a bit, hoping that this would solve some problems (or at least reduce technical debt). however, I'm afraid this causes more work than it solves problems for now.
• Details for the float texture problem:
  • the console is spammed with GL_INVALID_OPERATION: Level of detail outside of range. when the look up texture is updated
  • I tried to switch to signed integer textures, but the shader still only finds zeros in the texture apparently
  • I also tried to use another format than LuminanceAlpha for the two-channel format (I tried RGFormat which seems to be new to webgl2). not much luck, either, though.
  • I suspect that the implementation of UpdatableTexture could be at fault

[philippotto]

Closing in favor of #6748 which implements the basic idea of this PR, too.