missing compressed_segmentation_block_size with multiple mip levels

[xiuliren]

this only works when max_mip=0, if max_mip is larger than 0, it will have an error reporting that there are missing compressed_segmentation_block_size in other scales.

def test_write_compressed_segmentation():
  delete_layer()
  cv, data = create_layer(size=(128,64,64,1), offset=(0,0,0))

  cv.info['num_channels'] = 1
  cv.info['data_type'] = 'uint32'
  cv.scale['encoding'] = 'compressed_segmentation'
  cv.scale['compressed_segmentation_block_size'] = (8,8,8)
  cv.commit_info()

[william-silversmith]

Hi Jingpeng! I'll give this a look. Thanks for reporting!