Add custom mesh polygon support
The custom mesh boundary that was experimented with before the fuzzy input data update (#14 (closed)) was well received by users and should be re-added.
Background: This "new-style" mesh boundary polygon would trim the existing geometry to it and split segments as needed, as well as add boundary polygons that copied the elevation of the nearest existing data points.
It ensured that only points within the mesh boundary were part of the output mesh, and could be expanded to support polygon holes as well (which would allow cutting out islands from an elevation mesh to be interpolated with DEM data, among other things).