UV-mapper doesn't work properly

[Nusiq]

I moved this to another issue because the problem (from #42). I originally thought that the exporter is the problem but in reality it was the UV-mapper.

The problem reappeared. Here is some data I used for testing:
blender-project-model-and-expected-results.zip

Here are the screenshots from blockbench. Per-face uv-mapping exported from blender (pink side is the front on):

After changing the UV-mapping manually to standard uv-mapping here is how it looks:

Bottom side wasn't mapped propertly so the exporter used the per-face UV-mapping because the shape was impossible to export with standard UV-mapping.

Originally posted by @Nusiq in #42 (comment)

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UV-mapping in blender. Matches the images displayed in blockbench which means that the exporter works fine but UV-mapper doesn't work properly:

Originally posted by @Nusiq in #42 (comment)