Fix/crashing when generating walls (#6325)

* Fix crash when filling very tiny (1-5nm) gaps (#6279)

Cherry-picked from prusa3d/PrusaSlicer@8784ca0ecf

Co-authored-by: Lukáš Hejl <hejl.lukas@gmail.com>

* SPE-2256: Fix the issue that we used the old Voronoi graph during the detection of invalid Voronoi diagrams.

This happens because we didn't set a modified flag that is required to be set before we use the new Voronoi graph.
Possibly related to #12385

Cherry-picked from prusa3d/PrusaSlicer@2de1f3aa45

Co-authored-by: Lukáš Hejl <hejl.lukas@gmail.com>

---------

Co-authored-by: Lukáš Hejl <hejl.lukas@gmail.com>

src/libslic3r/Geometry/Voronoi.cpp

@@ -146,6 +146,9 @@ void VoronoiDiagram::copy_to_local(voronoi_diagram_type &voronoi_diagram) {
             new_edge.prev(&m_edges[prev_edge_idx]);
         }
     }
+
+    m_voronoi_diagram.clear();
+    m_is_modified = true;
 }
 
 template<typename SegmentIterator>
@@ -346,9 +349,6 @@ VoronoiDiagram::try_to_repair_degenerated_voronoi_diagram_by_rotation(const Segm
     for (vertex_type &vertex : m_vertices)
         vertex.color(0);
 
-    m_voronoi_diagram.clear();
-    m_is_modified = true;
-
     return issue_type;
 }