Implement `Geometry::flatten_to_2d`

[lnicola]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

[lnicola]

r? @metasim

[lnicola]

r? @jdroenner

[metasim]

bors r+

[bors[bot]]

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• ci gdal-3.1.3
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