Currently, the Kartographer SimpleStyleParser will translate geomask external data into a geoshape request, and then the mapdata library runs a transformation on the result to reverse the shapes as a mask. This could be supported directly by a new geomask endpoint in kartotherian, which would consolidate our handling in one code base, making the feature more reliable and easier to test and maintain.

Resolve whether we want to copy properties for geomasks. It's questionable, for example a border color might make sense but a description would not.