Speed up mapping creation/activation

[daniel-wer]

This PR speeds up mapping creation/activation of large mappings substantially (~5-6x faster). Key optimizations are:

• avoid resizing of ArrayBuffers by allocating a buffer padded to the correct size right at the start
• avoid calling Object.keys (and _.size(...) for that matter), as it is really slow for large objects, by keeping track of the keys ourselves

Example: Activating the astrocyte-full mapping went from taking 41.1s overall to taking 8.7s overall for me. The time to create the texture went from taking 23.3s to taking 3.8s.

I added a screenshot test for mappings while I was at it :)

URL of deployed dev instance (used for testing):

• https://speedupmappingcreation.webknossos.xyz

Steps to test:

• Open a tracing and use the mapping UI to activate a mapping. Everything should work and it should be noticeably faster than on master.
• Use the api to activate a mapping: webknossos.apiReady().then(async api => api.data.setMapping("segmentation", { <visible_id>: 0}))

---

• Updated changelog
• Ready for review

[philippotto]

Excellent PR! Very cool that you found the culprit plus a nice workaround for it 👍 And the additional screenshot test is the cherry on top 🍒

[philippotto]

Nice catch / improvement!