GEOSGeom_setPrecision flattens multi-geometries

[pramsey]

So no-op precision reductions end up having a spurious type change as well:

geosop -a "MULTILINESTRING((0 0,10 10))" -f wkt reducePrecision 1

LINESTRING (0 0, 10 10)

[pramsey]

@dr-jts thoughts?

[pramsey]

Reported in https://trac.osgeo.org/postgis/ticket/5208

[dr-jts]

It seems reasonable to preserve the collection type of the input.