Crop polygon properly

[azhavoro]

Motivation and context

Resolve #2992

How has this been tested?

Manually

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage increased (+0.06%) to 75.009% when pulling 7cbd359 on az/fix_crop into f5277f9 on develop.

[bsekachev]

Hi @dvkruchinin

Need a test for this bug. Let me know if you need any assistance.