Support compressed segmentation for Neuroglancer precomputed datasets

[frcroth]

#6716 does not support compressed segmentation: https://github.com/google/neuroglancer/tree/master/src/neuroglancer/sliceview/compressed_segmentation.

This format is used in all (?) segmentations, so currently segmentations can not be viewed.

There are two implementations: TS (https://github.com/google/neuroglancer/tree/master/src/neuroglancer/sliceview/compressed_segmentation) and C++ (https://github.com/seung-lab/compressedseg) with Python and Go bindings. There is no Java/Scala library.