Geom: interate ClipperLib::Path by reference

ClipperLib::Path is std::vector<IntPoint>. Iterating this with
"for( auto point : path)" could result in 'n' IntPoint copy-constructions.
It does seem GCC 8, at least, manages to optimise these constructions
out.

Replace with the "standard" for( const auto& point : path) idiom.

include/geometry/shape_line_chain.h

@@ -132,7 +132,7 @@ public:
     {
         m_points.reserve( aPath.size() );
 
-        for( auto point : aPath )
+        for( const auto& point : aPath )
             m_points.emplace_back( point.X, point.Y );
     }