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/SupportMaterial.cpp

@@ -1354,9 +1354,9 @@ struct SupportAnnotations
         object.project_and_append_custom_facets(false, EnforcerBlockerType::BLOCKER, blockers_layers);
     }
 
-    std::vector<ExPolygons>         enforcers_layers;
-    std::vector<ExPolygons>         blockers_layers;
-    const std::vector<Polygons>&    buildplate_covered;
+    std::vector<Polygons>         enforcers_layers;
+    std::vector<Polygons>         blockers_layers;
+    const std::vector<Polygons>&  buildplate_covered;
 };
 
 struct SlicesMarginCache