interpolations: snap lines to points, not vice versa

Avoids the occasional rounding problem which might occur when splitting
a line anywhere but on a support point, see postgis doc for ST_Split.

Fixes #253

sql/postgis_20_aux.sql

@@ -4,7 +4,7 @@ CREATE OR REPLACE FUNCTION split_line_on_node(line GEOMETRY, point GEOMETRY)
 RETURNS GEOMETRY
   AS $$
 BEGIN
-  RETURN ST_Split(line, ST_ClosestPoint(line, point));
+  RETURN ST_Split(ST_Snap(line, point, 0.0005), point);
 END;
 $$
 LANGUAGE plpgsql;