Stop using centroids for public_transport and pois tables

https://github.com/pnorman/spirit/blob/b4168097384be34da31c41bc616114cceaec24dd/themes/shortbread/topics/public_transport.lua#L96-L102 https://github.com/pnorman/spirit/blob/b4168097384be34da31c41bc616114cceaec24dd/themes/shortbread/topics/pois.lua#L187-L194 This leads to labels not inside polygons