Fix missing polygons when using simplify

When simplifying polygons, more polygons could be returned than
initially. The new simplify implementation didn't handle this case, and
polygons could be lost. We now resize the list to the proper polygons
count.

CURA-9830

src/geometry/Shape.cpp

@@ -842,16 +842,20 @@ void Shape::simplify(ClipperLib::PolyFillType fill_type)
     }
 
     // This is the actual content from clipper.cpp::SimplifyPolygons, but rewritten here in order
-    // to avoid a list copy
+    // to avoid having to put all the polygons in a transitory list
     ClipperLib::Clipper clipper;
     ClipperLib::Paths ret;
     clipper.StrictlySimple(true);
     addPaths(clipper, ClipperLib::ptSubject);
     clipper.Execute(ClipperLib::ctUnion, ret, fill_type, fill_type);
 
-    for (size_t i = 0; i < size(); ++i)
+    resize(ret.size());
+
+    for (size_t i = 0; i < ret.size(); i++)
     {
-        getLines()[i].setPoints(std::move(ret[i]));
+        Polygon& polygon = getLines()[i];
+        polygon.setExplicitelyClosed(clipper_explicitely_closed_); // Required for polygon newly created by resize()
+        polygon.setPoints(std::move(ret[i]));
     }
 }