Fix segmentation tab

[philippotto]

The segmentation value was always requested for zoomstep 0. If there weren't buckets at the requested location, no value could be found. This PR ensures that the value is requested for the current zoomstep (which mirrors the rendered data)

Mailable description of changes:

• Bug fix: Cell IDs are shown in the segmentation tab for all zoomsteps now
• The shortcut to copy the currently hovered cell id (CTRL + I) works in non-volume-tracings, too.

URL of deployed dev instance (used for testing):

• https://fixsegmentationtab.webknossos.xyz/

Steps to test:

• hover cells in datasets/tracings with segmentation (view, skeleton trace, volume trace) and use shortcut ctrl + I to copy the id
• open a dataset/tracing without segmentation and ensure that the shortcut doesn't crash (should be a noop)

Issues:

• fixes Segmentation tab doesn't always show correct values #2674

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• Ready for review

[daniel-wer]

The shortcut works well for me, however, I still feel like the reported segment IDs are not correct, when in another zoomstep than 0. Zoom out a little bit until the "downsampled" warning is displayed and then hover around in a cell. Most of the times, this will display a lot of different segment IDs in the segmentation tab for me, although the highlighted cell is always the same. :/

[daniel-wer]

I can demonstrate if you have trouble to reproduce :)

[daniel-wer]

Fixed the voxelIndex computation (by using the correct resolution), works well now :)