code: function setShared (shared) {
    let polygons = this.polygons.map(function (p) {
      return new Polygon3(p.vertices, shared, p.plane)
    })
    let result = fromPolygons(polygons)
    result.properties = this.properties // keep original properties
    result.isRetesselated = this.isRetesselated
    result.isCanonicalized = this.isCanonicalized
    return result
  }
docs: set the .shared property of all polygons
  @param  {Object} shared
  @returns {Geom3} Returns a new Geom3 solid, the original is unmodified!