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/SLA/Hollowing.cpp

@@ -286,8 +286,6 @@ void cut_drainholes(std::vector<ExPolygons> & obj_slices,
     
     if (mesh.empty()) return;
     
-    mesh.require_shared_vertices();
- 
     std::vector<ExPolygons> hole_slices = slice_mesh_ex(mesh.its, slicegrid, closing_radius, thr);
     
     if (obj_slices.size() != hole_slices.size())
@@ -316,7 +314,6 @@ void hollow_mesh(TriangleMesh &mesh, const Interior &interior, int flags)
         remove_inside_triangles(mesh, interior);
 
     mesh.merge(TriangleMesh{interior.mesh});
-    mesh.require_shared_vertices();
 }
 
 // Get the distance of p to the interior's zero iso_surface. Interior should
@@ -557,8 +554,7 @@ void remove_inside_triangles(TriangleMesh &mesh, const Interior &interior,
     new_faces = {};
 
     mesh = TriangleMesh{mesh.its};
-    mesh.repaired = true;
-    mesh.require_shared_vertices();
+    //FIXME do we want to repair the mesh? Are there duplicate vertices or flipped triangles?
 }
 
 }} // namespace Slic3r::sla