api: fix corner case staggered fd for centered x0

[mloubout]

Needed for elastic tti that contains odd centering FD

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Codecov Report

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

Project coverage is 86.76%. Comparing base (9e220ae) to head (28b9a25).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2373 +/- ## ======================================= Coverage 86.76% 86.76% ======================================= Files 233 233 Lines 43741 43764 +23 Branches 8075 8079 +4 ======================================= + Hits 37953 37973 +20 - Misses 5079 5080 +1 - Partials 709 711 +2 ```

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[FabioLuporini]

thanks, nice cleanup, merging