Make color layers transparent if color value is out of range

[MichaelBuessemeyer]

This PR lets cover layers "shine / see through" for each voxel where the color value is out of the range configured as the histogram min and max bounds. This setting can be adjusted by the user and therefore this feature is independent of the exact color layer value range.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Open a dataset with multiple color layers. Preferably one with affine transformations.
• Turn on cover blend mode in the settings tab.
• In the layer settings adjust the min & max settings of the histogram (can be turned on via the menu in the top right corner of the histogram view)
• The lower color layers should now shine through the upper color layer

Issues:

• No issue exists for this.

---

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

• [ ] Updated changelog