make_renderable.hh contains definitions for make_renderable(std::vector<std::pair<Pos3, Pos3>> const& pos) and make_renderable(std::vector<std::array<Pos3, 2>> const& pos) which both simple call viewer::lines(pos).
viewer::lines calls make_mesh_definition(pos) which did not exist for std::vector<std::pair<Pos3, Pos3>> const& and was using a wrong implementation for std::vector<std::array<Pos3, 2>> const& pos (which failed trying to add faces with only two vertices).