pcbnew: Don't copy const structures when not needed

Copying the const pointer on large structures (filled zones) will be
very slow and memory intensive.  This can cause OOM on linux and extreme
slowdowns when the save is triggered.

Fixes: lp:1840168
* https://bugs.launchpad.net/kicad/+bug/1840168

include/geometry/shape_poly_set.h

@@ -171,7 +171,7 @@ class SHAPE_POLY_SET : public SHAPE
                 if( m_currentPolygon != m_poly->OutlineCount() - 1 )
                     return false;
 
-                auto currentPolygon = m_poly->CPolygon( m_currentPolygon );
+                const auto& currentPolygon = m_poly->CPolygon( m_currentPolygon );
 
                 return m_currentContour < (int) currentPolygon.size() - 1
                            || m_currentVertex < currentPolygon[m_currentContour].PointCount();