[ogr] Fix mapinfo (multi)polygon type layers

[elpaso]

By not flattening the geometry, it applies to all mixed geometry OGR layers

Fixes #60396

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[nyalldawson]

@elpaso I dug DEEP into the archives, and the behavior is coming from c8ff386

Which is a fix for https://web.archive.org/web/20240715001715/https://issues.qgis.org/issues/8001. note that there's NO tests for that issue/fix, so we should add those before continuing.

That said, I think a cleaner fix would be to remove the single type conversion in the iteration loop entirely, and then implement the desired logic afterwards -- ie just before https://github.com/qgis/QGIS/blob/master/src/core/providers/ogr/qgsogrproviderutils.cpp#L2758

[elpaso]

@elpaso I dug DEEP into the archives, and the behavior is coming from c8ff386

Which is a fix for https://web.archive.org/web/20240715001715/https://issues.qgis.org/issues/8001. note that there's NO tests for that issue/fix, so we should add those before continuing.

That said, I think a cleaner fix would be to remove the single type conversion in the iteration loop entirely, and then implement the desired logic afterwards -- ie just before https://github.com/qgis/QGIS/blob/master/src/core/providers/ogr/qgsogrproviderutils.cpp#L2758

I refactored the whole types collapsing logic, can you please have a look?