Do not load data for disabled or invisible layers

[daniel-wer]

/cc @MichaelBuessemeyer This is somewhat relevant for your issue #4044 as well, so you could have a look at this PR as well :)

Also, I fixed a bug where the shader crashed when there were multiple float layers 🙈 (Will be obsolete with the Histogram PR as well, though) :)

URL of deployed dev instance (used for testing):

• https://savebandwidth.webknossos.xyz

Steps to test:

• Open a dataset with many layers, disable some of them or set alpha to 0. Move around - data should quicker than when all layers were enabled and you should only see bucket requests for visible layers in the network tab.
• Open a dataset with a segmentation layer. Switch to arbitrary mode. - You should see no requests for segmentation buckets in the network tab.

Issues:

• fixes Do not load data for data layers with an alpha value of 0 #3671

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• Updated changelog
• Ready for review

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[philippotto]

Excellent PR! 👍 The code is much clearer now and I'm looking forward to the speed gains 🍬

I'll test when the CI passes ;)

[daniel-wer]

@philippotto Ready to test :)

[philippotto]

Works flawlessly 👍