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.

tests/libslic3r/test_marchingsquares.cpp

@@ -319,7 +319,6 @@ static void recreate_object_from_rasters(const std::string &objname, float lh) {
     mesh.translate(tr.x(), tr.y(), tr.z());
     bb = mesh.bounding_box();
     
-    assert(mesh.has_shared_vertices());
     std::vector<ExPolygons> layers = slice_mesh_ex(mesh.its, grid(float(bb.min.z()) + lh, float(bb.max.z()), lh));
     
     sla::RasterBase::Resolution res{2560, 1440};