[Feature] Add center alignments for draw_texts in OpencvBackendVisualizer

[AntDum]

Motivation

This pull request addresses a gap in the OpencvBackendVisualizer class's draw_texts method. Although the docstring mentions support for "bottom," "top," and "center" alignments, the current implementation lacks the ability to center-align text. This modification aims to rectify this limitation.

Modification

I've updated the draw_texts method to support center alignment for both horizontal and vertical positioning.

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.