CVAT-UI: Couple of fixes

[bsekachev]

Motivation and context

1. Fixed: Interpolation algorithm, array must be flatten
2. Fixed: When redraw polygon with enabled feature of autobordering previous points are visible

How has this been tested?

The PR fixes couple of bugs reported by internal team and our community

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

Resolve #1882
Resolve #1886

[coveralls]

Pull Request Test Coverage Report for Build 6463

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 64.928%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/dataset_manager/annotation.py	1	2	50.0%

Totals
Change from base Build 6461:	0.01%
Covered Lines:	11035
Relevant Lines:	16588

---

💛 - Coveralls

[bsekachev]

@ActiveChooN @zhiltsov-max Could you please look at the PR? It is quite critical

[zhiltsov-max]

I think, it may be better to use some kind of search tree here for better performance. For instance, you can create a list of id(obj) and put it into a set.