Fix of #8474 and #8514: Voronoi generator sometimes produced a non-planar Voronoi diagram.

We introduced detecting for those degeneration cases. When degenerated Voronoi diagram is detected, then the input polygons are rotated, and the Voronoi diagram is recomputed. Usually, rotation of input data solves issues like this.

tests/libslic3r/test_voronoi.cpp

@@ -5,6 +5,7 @@
 #include <libslic3r/Polyline.hpp>
 #include <libslic3r/EdgeGrid.hpp>
 #include <libslic3r/Geometry.hpp>
+#include <libslic3r/Arachne/SkeletalTrapezoidation.hpp>
 
 #include <libslic3r/Geometry/VoronoiOffset.hpp>
 #include <libslic3r/Geometry/VoronoiVisualUtils.hpp>
@@ -2158,3 +2159,34 @@ TEST_CASE("Intersecting Voronoi edges", "[Voronoi]")
 
 //    REQUIRE(!has_intersecting_edges(poly, vd));
 }
+
+// In this case resulting Voronoi diagram is not planar. This case was distilled from GH issue #8474.
+// Also, in GH issue #8514, a non-planar Voronoi diagram is generated for several polygons.
+// Rotating the input polygon will solve this issue.
+TEST_CASE("Non-planar voronoi diagram", "[VoronoiNonPlanar]")
+{
+    Polygon poly {
+        { 5500000, -42000000},
+        { 8000000, -17000000},
+        { 8000000,  40000000},
+        { 7500000,  40000000},
+        { 7500000, -18000000},
+        { 6000001, -18000000},
+        { 6000000,  40000000},
+        { 5500000,  40000000},
+    };
+
+//    poly.rotate(PI / 6);
+
+    REQUIRE(poly.area() > 0.);
+    REQUIRE(intersecting_edges({poly}).empty());
+
+    VD    vd;
+    Lines lines = to_lines(poly);
+    construct_voronoi(lines.begin(), lines.end(), &vd);
+#ifdef VORONOI_DEBUG_OUT
+    dump_voronoi_to_svg(debug_out_path("voronoi-non-planar-out.svg").c_str(), vd, Points(), lines);
+#endif
+
+//    REQUIRE(Arachne::is_voronoi_diagram_planar(vd));
+}