text: Title: uint16 SegmentationMapOnImage
  Body: I think it is quite common to encode masks using uint16, therefore I would suggest to enable this format for the `SegmentationMapOnImage` class.
  
  It seems like this format is not supported yet: https://github.com/aleju/imgaug/blob/69ac72ef4f2b9d5de62c7813dcf3427ca4a604b5/imgaug/imgaug.py#L4814