Fix invalid polygons in input data set

[christophfink]

Some input data sets, including some versions of the Natural Earth adm0 layers, seem to contain invalid polygons, and it is still surprisingly easy to produce invalid geometries, e.g., when reprojecting data sets.

Let’s apply a zero-width buffer to any input data set in order to fix the most common problems. This is not very costly in terms of computation, and helps avoid frustration down the road.