Eradicated admesh from TriangleMesh:

TriangleMesh newly only holds indexed_triangle_set and
TriangleMeshStats. TriangleMeshStats contains an excerpt of stl_stats.
TriangleMeshStats are updated when initializing with indexed_triangle_set.

Admesh triangle mesh fixing is newly only used when loading an STL.
AMF / 3MF / OBJ file formats are already indexed triangle sets, thus
they are no more converted to admesh stl_file format, nor fixed
through admesh repair machinery. When importing AMF / 3MF / OBJ files,
volume is calculated and if negative, all faces are flipped. Also
a bounding box and number of open edges is calculated.

Implemented its_number_of_patches(), its_num_open_edges()
Optimized its_split(), its_is_splittable() using a visitor pattern.

Reworked QHull integration into TriangleMesh:
    1) Face normals were not right.
    2) Indexed triangle set is newly emitted instead of duplicating
       vertices for each face.

Fixed cut_mesh(): Orient the triangulated faces correctly.

src/libslic3r/SimplifyMesh.hpp

@@ -14,10 +14,8 @@ void simplify_mesh(indexed_triangle_set &);
 
 template<class...Args> void simplify_mesh(TriangleMesh &m, Args &&...a)
 {
-    m.require_shared_vertices();
     simplify_mesh(m.its, std::forward<Args>(a)...);
-    m = TriangleMesh{m.its};
-    m.require_shared_vertices();
+    m = TriangleMesh{ std::move(m.its) };
 }
 
 } // namespace Slic3r