Fix rendering for intensity range of 0

[MichaelBuessemeyer]

This PR fixes a bug where having one layer with the intensity range of 0 led to no data at all being rendered.

This problem is tackled by

1. determining min == max
2. if min == max add +1 to the divisor of the division to avoid dividing by 0
3. Later replace the calculated color value with black in the case of min == max.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Please do the following steps for a few datasets to ensure this fix works for "all". I tested this on the ROI and l4 sample dataset.
• Open a dataset in view mode
• Go to the layer settings and move the slider of a color layer of your choice to the same values (e.g. 100).
• The data layer should now no longer be rendered but the other active layers should still be visible.
• Additionally, there should be a warning icon stating that the layer is not rendered and that the user should prefer to disable the layer instead of reducing the intensity range to 0.

Issues:

• fixes Setting min and max to same value in histogram "hides" all layers #5671

---

• Updated (unreleased) changelog
• [x] Updated (unreleased) migration guide if applicable
• [ ] Updated documentation if applicable
• [ ] Adapted wk-connect if datastore API changes
• [ ] Needs datastore update after deployment
• Ready for review

[philippotto]

Great fix :)

[philippotto]

please update the changelog before merging :)