This needs an extension to the current GEOS_INIT_THEADS / GEOS_FINISH_THREADS mechanism that is already used for eg spatial predicates like intersects etc (ufuncs that do not return geometries).

For ufuncs returning geometries (eg the YY_Y ufuncs), this needs some additional logic, possibly a two-step loop (first loop creating the GEOSGeometry objects, second loop wrapping them in python objects). See also #113 (comment) for a brief discussion about this.