Renamed create_face_neighbors_index() to its_face_edge_ids().

Renamed its_create_neighbors_index() / its_create_neighbors_index_par() to its_face_neighbors() / its_face_neighbors_par().
New variant of its_face_edge_ids() to create edge IDs from face neighbors.
Fixed some incorrect use of _NDEBUG, it should be NDEBUG.
PrintObject::slice_support_volumes() returns newly Polygons, which are cheaper than ExPolygons.
Updated SeamPlacer and SupportMaterial to use regions defined as Polygons, not ExPolygons.
TriangleSelector::get_facets_strict() returning a patch with T-joints retriangulated.
New slice_mesh_slabs() - slicing projections of a triangle patch into top / bottom layers of slices, for MMU top / bottom segmentation.
TriangleMeshSlicer - use 64 mutexes instead of one when scattering sliced triangles into layers. This makes a big difference on modern many core desktop computers.
When applying MM segmented regions to input regions, the split regions are now re-merged with 10x higher positive offset epsilon to avoid creating gaps.
When testing for existence of paint-on supports or seam, use a more efficient has_facets() test, which does not deserialize into the expensive TriangleSelector tree structure.
GLIndexedVertexArray newly uses Eigen::AlignedBox<float, 3> for efficiency instead of our double based BoundingBoxf3.
Improved MMU painting refresh speed by optimizing generation of the vertex buffers.
Refactored MMU segmentation - projection of painted surfaces from top / bottom.
	1) Parallelized.
	2) Using the new slice_mesh_slabs() instead of projecting one triangle by the other and merging them with Clipper.

src/libslic3r/TriangleMeshSlicer.hpp

@@ -77,6 +77,21 @@ inline std::vector<ExPolygons>  slice_mesh_ex(
     return slice_mesh_ex(mesh, zs, params, throw_on_cancel);
 }
 
+// Slice a triangle set with a set of Z slabs (thick layers).
+// The effect is similar to producing the usual top / bottom layers from a sliced mesh by 
+// subtracting layer[i] from layer[i - 1] for the top surfaces resp.
+// subtracting layer[i] from layer[i + 1] for the bottom surfaces,
+// with the exception that the triangle set this function processes may not cover the whole top resp. bottom surface.
+// top resp. bottom surfaces are calculated only if out_top resp. out_bottom is not null.
+void slice_mesh_slabs(
+    const indexed_triangle_set       &mesh,
+    // Unscaled Zs
+    const std::vector<float>         &zs,
+    const Transform3d                &trafo,
+    std::vector<Polygons>            *out_top,
+    std::vector<Polygons>            *out_bottom,
+    std::function<void()>             throw_on_cancel);
+
 void                            cut_mesh(
     const indexed_triangle_set      &mesh,
     float                            z,