Geometry subtype not inferred for function returns

With
```
"postgraphile": "4.12.2",
"@graphile/postgis": "0.1.0",
```

The following function will return a geometry interface as a return type instead of a point geometry. This is particularly frustrating for computed columns on tables (for example generating a centroid for a polygon geometry).

```pgsql
CREATE FUNCTION myschema.mytable_pos(mytable myschema.mytable) RETURNS geometry(Point,4326) AS $$
SELECT st_pointonsurface(geom)
$$ LANGUAGE sql STABLE
````