Optimize masks 2

[zhiltsov-max]

Summary

Follow-up for #101

• Further optimized mask merging / exporting (10k x 10k image & 250 masks test, 80s -> 60s)
• Masks, covered by other masks with class = 0 (typically - background) are now exported correctly with holes inside

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT