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/Print.cpp

@@ -655,10 +655,8 @@ std::string Print::validate(std::string* warning) const
             // Notify the user in that case.
             if (! object->has_support() && warning) {
                 for (const ModelVolume* mv : object->model_object()->volumes) {
-                    bool has_enforcers = mv->is_support_enforcer()
-                        || (mv->is_model_part()
-                            && ! mv->supported_facets.empty()
-                            && ! mv->supported_facets.get_facets(*mv, EnforcerBlockerType::ENFORCER).indices.empty());
+                    bool has_enforcers = mv->is_support_enforcer() || 
+                        (mv->is_model_part() && mv->supported_facets.has_facets(*mv, EnforcerBlockerType::ENFORCER));
                     if (has_enforcers) {
                         *warning = "_SUPPORTS_OFF";
                         break;