Fix initial entity position in exporter mappings

[soininen]

Fixes spine-tools/Spine-Toolbox#2530

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

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

Comparison is base (102b81c) 60.12% compared to head (9457391) 60.02%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #183 +/- ## =========================================== - Coverage 60.12% 60.02% -0.10% =========================================== Files 194 180 -14 Lines 17878 17697 -181 Branches 2922 2899 -23 =========================================== - Hits 10749 10623 -126 + Misses 6555 6499 -56 - Partials 574 575 +1 ```

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