[QGIS-Developer] PSA: do NOT build QGIS with libspatialindex >= 2.0

[Nyall Dawson]

Hi list,

While tracking down the failures from
https://github.com/qgis/QGIS/pull/63180, it's revealed that using QGIS with
libspatialindex >= 2.0 gives completely misleading results for nearest
neighbour geometry searches.

The root cause is an optimisation from the upstream library (
https://github.com/libspatialindex/libspatialindex/commit/6fe9ff769243579ed74f7f27409dd1dda6591634)
that breaks how downstream clients could previously implement custom
nearest neighbour comparators. From my research I don't think this is
fixable in QGIS, and will need an upstream fix.

In the meantime, any QGIS build with libspatialindex >= 2 will be silently
broken and give COMPLETELY INCORRECT results for anything based on a
nearest neighbour query.

This also affects downstream clients like QField/mergin.

Do NOT use in production -- you have been warned!!

Nyall
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