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_aabbindirect.cpp

@@ -9,7 +9,6 @@ using namespace Slic3r;
 TEST_CASE("Building a tree over a box, ray caster and closest query", "[AABBIndirect]")
 {
     TriangleMesh tmesh = make_cube(1., 1., 1.);
-    tmesh.repair();
 
     auto tree = AABBTreeIndirect::build_aabb_tree_over_indexed_triangle_set(tmesh.its.vertices, tmesh.its.indices);
     REQUIRE(! tree.empty());