merge: Handle projection better

[m-mohr]

One thing we will run in to is the projection, especially with EPSG:4326 (WGS84) as default. Geometries can be transformed with a bijective function from source to target projection, but projected geometries can yield different validity results (e.g. intersecting lines). We will solve that as we encounter that.

Originally posted by @ivorbosloper in #102 (review)