Fix crashes when using extra infill support lines

CURA-12232
Some references were created to temporary objects, which most of the
times work but sometimes crash if the memory was reallocated at the
point the object is being used.

src/FffGcodeWriter.cpp

@@ -2186,7 +2186,7 @@ void wall_tool_paths2lines(const std::vector<std::vector<VariableWidthLines>>& w
         {
             for (const ExtrusionLine& c : b)
             {
-                const Polygon& poly = c.toPolygon();
+                const Polygon poly = c.toPolygon();
                 if (c.is_closed_)
                 {
                     result.push_back(poly.toPseudoOpenPolyline());
@@ -2343,12 +2343,12 @@ void addExtraLinesToSupportSurfacesAbove(
     // invert the supported_area by adding one huge polygon around the outside
     supported_area.push_back(AABB{ supported_area }.toPolygon());
 
-    const Shape& inv_supported_area = supported_area.intersection(part.getOwnInfillArea());
+    const Shape inv_supported_area = supported_area.intersection(part.getOwnInfillArea());
 
     OpenLinesSet unsupported_line_segments = inv_supported_area.intersection(printed_lines_on_layer_above);
 
     // This is to work around a rounding issue in the shape library with border points.
-    const Shape& expanded_inv_supported_area = inv_supported_area.offset(-EPSILON);
+    const Shape expanded_inv_supported_area = inv_supported_area.offset(-EPSILON);
 
     Simplify s{ MM2INT(1000), // max() doesnt work here, so just pick a big number.
                 infill_line_width,
@@ -2358,7 +2358,7 @@ void addExtraLinesToSupportSurfacesAbove(
 
     for (const OpenPolyline& a : unsupported_line_segments)
     {
-        const OpenPolyline& simplified = s.polyline(a);
+        const OpenPolyline simplified = s.polyline(a);
         for (const Point2LL& point : simplified)
         {
             size_t idx = expanded_inv_supported_area.findInside(point);