Gerbview: fix incorrect bounding box of lines drawn with a rectangular aperture.

Fixes #9152
https://gitlab.com/kicad/code/kicad/issues/9152

gerbview/gerber_draw_item.cpp

@@ -387,6 +387,9 @@ const EDA_RECT GERBER_DRAW_ITEM::GetBoundingBox() const
     {
         if( code && code->m_Shape == APT_RECT )
         {
+            if( m_Polygon.OutlineCount() == 0 )
+                ConvertSegmentToPolygon();
+
             if( m_Polygon.OutlineCount() > 0 )
             {
                 auto bb = m_Polygon.BBox();