Add MOTS png format

[zhiltsov-max]

Motivation and context

Depends on openvinotoolkit/datumaro#21

Closes openvinotoolkit/datumaro#15
Closes #2129

• Adds MOTS png format for polygon and bbox tracks
• Added support of skips in imported tracks in MOT and MOTS formats, the skips produce outside frames in tracks
• Fixes export of tracks - tracks could be truncated (Tracks end on last key frame in export even though they are shown in CVAT UI #2129)

How has this been tested?

Unit tests, manual test

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage increased (+1.4%) to 66.695% when pulling 8b710b6 on zm/add-mots-format into 476318f on develop.

[nmanovic]

@Marishka17 , could you please review the patch? Provide your comments, resolve conflicts, test the patch.

[bsekachev]

Hi, @Marishka17
Can I ask you to look at the code and test this patch?