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/SLAPrintSteps.cpp

@@ -526,7 +526,6 @@ void SLAPrint::Steps::slice_model(SLAPrintObject &po)
     }
     auto  thr        = [this]() { m_print->throw_if_canceled(); };
     auto &slice_grid = po.m_model_height_levels;
-    assert(mesh.has_shared_vertices());
     po.m_model_slices = slice_mesh_ex(mesh.its, slice_grid, params, thr);
 
     sla::Interior *interior = po.m_hollowing_data ?