Fix invalid polygons after simplification, coord. transform or clipping

[pka]

Some vector tile creation steps can produce invalid polygons:

• Simplification
• Coordinate transform
• Clipping (tile buffer)

See also mapbox/vector-tile-spec#58 (comment) and tilezen/mapbox-vector-tile#42 (comment)

[Wykks]

Indeed. I'm getting this

Using these data https://raw.githubusercontent.com/gregoiredavid/france-geojson/v2-hd-dev/departements.geojson
Imported to postgis with
ogr2ogr -f PostgreSQL PG:"host=127.0.0.1 port=9500 dbname=postgres user=postgres password=test" departements.geojson -nln depts -s_srs EPSG:4326 -t_srs EPSG:3857

Edit:
Oops, it's actually working fine with buffer_size = 1 (after reading what going on in #18)
For some reason, genconfig didnt set buffer_size = 1 for this layer.
It set geometry_type = "GEOMETRY" which doesn't works. So I changed to POLYGON but forgot to set buffer_size = 1

[pka]

@Wykks buffer_size is now also generated for type GEOMETRY (eb89553)

[Wykks]

Actually, I don't know what's type GEOMETRY. T-rex generate config with this, but then it doesn't know what to do with it:
Error converting between Postgres and Rust types: Unknown geometry type GEOMETRY.
It seems that t-rex is incorrectly detecting the geometry_type from postgis.
QGIS detect the geometry_type POLYGON without issue, so I think that my datasource is fine.