Fix mask export when background color has nonzero index in Cityscapes and KITTI

[zhiltsov-max]

Motivation and context

Fixes #5031

Follow-up for #5056, fixes mask export for other formats that were not covered by #5056

How has this been tested?

Unit tests

Checklist

• I submit my changes into the develop branch
• I have added a 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.