[DAR-1104][External] Fix colours mismatch between files on semantic mask export

[AndriiKlymchuk]

Problem

The same semantic mask could be represented by different colors in the files of the same export.

Solution

The root cause of the issue is an assumption that labels in the raster_layer.data.mask_annotation_ids_mapping are the same across different files which is not true. Because of that assumption labels were used as colors ids causing the mismatch.
I changed the logic to have static color ids for all files and convert labels into color ids for every file separately.

Changelog

Fix colours mismatch between files on semantic mask export

[linear]

DAR-1104 BUG: Inconsistent mapping between classes and colours in semantic mask export

[JBWilkie]

Tested by comparing output masks of master branch to this one, always had consistent colour mappings on this branch

LGTM!

[saurbhc]

Thank you 🙏

[AndriiKlymchuk]

@umbertoDifa you were to fast. I found one issue during testing. Opening another PR.