Feature request: clipping mesh by (Multi)Polygon with holes

[grongen]

A GeometryList can contain a representation of a MultiPolygon or Polygon with holes. The function mkernel_mesh2d_delete now only seems to work for a Polygon without holes (a GeometryList with -999.0 or -998.0 is not processed). It would be convenient if for example a rectilinear mesh can be clipped by a Multipolygon. Now problems arise when generating a mesh with islands, or even worse, things like islands with lakes.