Fixing projection of bottom surfaces in MM segmentation and for

support blockers / enforcers.

All slicing functions shall produce consistent results with the same mesh, same transformation matrix and slicing parameters.
Namely, slice_mesh_slabs() shall produce consistent results with slice_mesh() and slice_mesh_ex() in the sense, that projections made by
slice_mesh_slabs() shall fall onto slicing planes produced by slice_mesh().

Before this commit, slice_mesh_slabs() projected bottom facing faces upwards to its coplanar slicing plane,
which is different from how slice_mesh() or slice_mesh_ex() work, leading to ignored support enforcer / blocker strokes.

src/libslic3r/PrintObject.cpp

@@ -2284,6 +2284,7 @@ void PrintObject::project_and_append_custom_facets(
                         seam, out);
                 else {
                     std::vector<Polygons> projected;
+                    // Support blockers or enforcers. Project downward facing painted areas upwards to their respective slicing plane.
                     slice_mesh_slabs(custom_facets, zs_from_layers(this->layers()), this->trafo_centered() * mv->get_matrix(), nullptr, &projected, [](){});
                     // Merge these projections with the output, layer by layer.
                     assert(! projected.empty());