Account for the padding shift during vectorization + use multipolygon

[quentinblampey]

In _vectorize_mask, we pad the mask before using find_contours, which is needed to have correct geometries. Yet, it create a shift of one pixel in the coordinates.

This PR shifts the geometry of one pixel to recover the original coordinates. In the test that I added, when doing rasterize + vectorize, we now have an IoU of 0.98 instead of 0.92.

When one label has multiple geometries, I now use a MultiPolygon instead of multiple Polygon. This is more convenient and less ambiguous, as we'll have only one row per label from the mask.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.09%. Comparing base (eb5a202) to head (ae6e571).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/_core/operations/vectorize.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #921      +/-   ##
==========================================
- Coverage   92.11%   92.09%   -0.02%     
==========================================
  Files          48       48              
  Lines        7429     7430       +1     
==========================================
  Hits         6843     6843              
- Misses        586      587       +1     

Files with missing lines	Coverage Δ
src/spatialdata/_core/operations/vectorize.py	93.12% <88.88%> (-0.59%)	⬇️

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[LucaMarconato]

Thanks for the PR! I leave my review.

[LucaMarconato]

If you agree on my changes we can merge.

[quentinblampey]

Thanks @LucaMarconato, your change looks good!