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/TriangleMeshSlicer.hpp

@@ -98,7 +98,21 @@ void slice_mesh_slabs(
     std::vector<Polygons>            *out_bottom,
     std::function<void()>             throw_on_cancel);
 
-void                            cut_mesh(
+// Project mesh upwards pointing surfaces / downwards pointing surfaces into 2D polygons.
+void project_mesh(
+    const indexed_triangle_set       &mesh,
+    const Transform3d                &trafo,
+    Polygons                         *out_top,
+    Polygons                         *out_bottom,
+    std::function<void()>             throw_on_cancel);
+
+// Project mesh into 2D polygons.
+Polygons project_mesh(
+    const indexed_triangle_set       &mesh,
+    const Transform3d                &trafo,
+    std::function<void()>             throw_on_cancel);
+
+void cut_mesh(
     const indexed_triangle_set      &mesh,
     float                            z,
     indexed_triangle_set            *upper,