Fix/crashing when generating walls (#6325)

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

Cherry-picked from prusa3d/PrusaSlicer@8784ca0

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@2de1f3a

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

---------

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

src/libslic3r/Geometry/MedialAxis.cpp

@@ -449,6 +449,19 @@ MedialAxis::MedialAxis(double min_width, double max_width, const ExPolygon &expo
 void MedialAxis::build(ThickPolylines* polylines)
 {
     m_vd.construct_voronoi(m_lines.begin(), m_lines.end());
+
+    // For several ExPolygons in SPE-1729, an invalid Voronoi diagram was produced that wasn't fixable by rotating input data.
+    // Those ExPolygons contain very thin lines and holes formed by very close (1-5nm) vertices that are on the edge of our resolution.
+    // Those thin lines and holes are both unprintable and cause the Voronoi diagram to be invalid.
+    // So we filter out such thin lines and holes and try to compute the Voronoi diagram again.
+    if (!m_vd.is_valid()) {
+        m_lines = to_lines(closing_ex({m_expolygon}, float(2. * SCALED_EPSILON)));
+        m_vd.construct_voronoi(m_lines.begin(), m_lines.end());
+
+        if (!m_vd.is_valid())
+            BOOST_LOG_TRIVIAL(error) << "MedialAxis - Invalid Voronoi diagram even after morphological closing.";
+    }
+
     Slic3r::Voronoi::annotate_inside_outside(m_vd, m_lines);
 //    static constexpr double threshold_alpha = M_PI / 12.; // 30 degrees
 //    std::vector<Vec2d> skeleton_edges = Slic3r::Voronoi::skeleton_edges_rough(vd, lines, threshold_alpha);

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;
 }