Address more remaining clang++ errors in Milena.
* mln/core/concept/proxy.hxx: Add and reorder forward declarations. * mln/accu/site_set/rectangularity.hh (rectangularity<P>): Do not re-import base class attributes as protected members, so that the base class can still access them. Adjust. * mln/geom/seeds2tiling_roundness.hh, * mln/subsampling/antialiased.hh, * mln/world/inter_pixel/dim2/make_edge_image.hh: Add missing header inclusions. * mln/io/plot/save.hh (save) * mln/morpho/tree/propagate_node.hh (propagate_node_to_descendants): Do not provide default arguments twice. * mln/topo/face_iter.hh, * mln/topo/n_face_iter.hh: Add `this->' qualifiers to disambiguate calls to base class methods. * tests/metal/converts_to.cc (from_2): Do not rely on uninitialized data. (main): Add a test case.