Fix scale parameter for polygons with datashader

[timtreis]

Summary

• Fixes scale parameter for shapes has no impact on polygons with datashader #473: the scale parameter in render_shapes() had no effect on Polygon/MultiPolygon geometries when using the datashader backend
• Adds scaling via shapely.affinity.scale() around each geometry's centroid in the datashader path, matching existing matplotlib behavior
• Adds regression test test_plot_can_scale_polygons_datashader

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.87%. Comparing base (61ed9c3) to head (b199ad0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #555      +/-   ##
==========================================
+ Coverage   73.66%   73.87%   +0.20%     
==========================================
  Files          10       10              
  Lines        2738     2752      +14     
  Branches      637      639       +2     
==========================================
+ Hits         2017     2033      +16     
+ Misses        447      446       -1     
+ Partials      274      273       -1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	82.12% <100.00%> (+0.24%)	⬆️

... and 4 files with indirect coverage changes

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