Improve Shape point converter

Summary

For the 100K generation of Polygon annotations, the time consumption was 1.69 secs to 1.46 secs (about 15% faster).

import_from	coco
before	34.77 s
after	31.53 s

How to test

Checklist

[ ] I have added unit tests to cover my changes.
[ ] I have added integration tests to cover my changes.
[ ] I have added the description of my changes into CHANGELOG.
[ ] I have updated the documentation accordingly

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) 2024 Intel Corporation
#
# SPDX-License-Identifier: MIT

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Please upload report for BASE (releases/1.7.0@294c0e5). Learn more about missing BASE report.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## releases/1.7.0 #1508 +/- ## ================================================= Coverage ? 80.77% ================================================= Files ? 275 Lines ? 31514 Branches ? 6358 ================================================= Hits ? 25456 Misses ? 4644 Partials ? 1414 ``` | [Flag](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1508/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | Coverage Δ | | |---|---|---| | [ubuntu-20.04_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1508/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `80.76% <ø> (?)` | | | [windows-2022_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1508/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `80.75% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit#carryforward-flags-in-the-pull-request-comment) to find out more.

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openvinotoolkit / datumaro