gdalwarp: be more robust to numerical instability when selecting overviews #12992

Job	Run time
build-mac	9m 13s
build-linux-ubuntu-focal	4m 5s
build-windows-conda	18m 40s
build-windows-minimum	18m 9s
build-windows-msys2-mingw	11m 34s
build-windows-conda-csharp-only	0s
	1h 1m 41s

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