Fix export of masks with multi-theC as multi-channel labels

[will-moore]

This commit is from #174 (being tested there by Jason) but is picked to a separate PR so we can merge and release...

If masks have theC set (and there is more than 1 value for theC) then we export a multi-channel mask image (don't squeeze out the C dimension).

cc @jburel

This bug was introduced in 101b0e1, released in v0.6.0 (June 2024)

To test, create or find an Image with Polygons that have different values for theC. E.g. on merge-ci Image:23117:

Export the image, then export polygons to labels, using --overlaps=dtype_max because some Shapes with the same C value are overlapping:

$ omero zarr export Image:23117
$ omero zarr polygons Image:23117 --overlaps=dtype_max

Now view with:

$ ome_zarr view 23117.ome.zarr/

Click on labels/0 link to view the labels in validator, then click the vizarr icon to view labels in vizarr.
You should see that this is a multichannel image, with labels on different channels.
By default, vizarr will scale rendering so that the brightest label (e.g. value 1 or value 2) is saturated. For any overlapping labels, the overlap is assigned to max, e.g. 127, so the other labels (e.g. value 1 or value 2) will be almost black, unless you boost the levels:

Without this PR, all the labels are exported to a single channel:

[snoopycrimecop]

Conflicting PR. Removed from build OMERO-plugins-push#516. See the console output for more details.
Possible conflicts:

• PR Use zarr v3 and updated ome-zarr-py #172 will-moore 'Use zarr v3 and updated ome-zarr-py'
  • test/integration/clitest/test_export.py

--conflicts Conflict resolved in build OMERO-plugins-push#517. See the console output for more details.

[snoopycrimecop]

Conflicting PR. Removed from build OMERO-plugins-push#535. See the console output for more details.
Possible conflicts:

• Upstream changes
  • CHANGELOG.md

--conflicts