Support OME-NGFF labels metadata

[krokicki]

When opening an OME-NGFF image, the labels metadata are currently ignored.

The following example uses this image from the IDR.

This Neuroglancer link will open this JSON:

{
  "layers": [
    {
      "name": "image",
      "type": "auto",
      "source": "https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0079A/idr0079_images.zarr/0"
    }
  ],
  "layout": "4panel-alt"
}

Recent improvements from @seankmartin now allow Neuroglancer to open all of the image channels in a very usable way. It would be great to also read any labels and add them as segmentation layers.