MeshBoolean: fix GCC 14 compile error

[FliegendeWurst]

Otherwise the build fails with:

/build/source/src/libslic3r/MeshBoolean.cpp:161:16: error: non-const lvalue reference to type 'reference' (aka 'CGAL::SM_Vertex_index') cannot bind to a temporary of type 'reference' (aka 'CGAL::SM_Vertex_index')
  161 |     for (auto &vi : cgalmesh.vertices()) {
      |                ^  ~
/build/source/src/libslic3r/MeshBoolean.cpp:188:12: note: in instantiation of function template specialization 'Slic3r::MeshBoolean::cgal::cgal_to_triangle_mesh<CGAL::Surface_mesh<CGAL::Point_3<CGAL::Epick>>>' requested here
  188 |     return cgal_to_triangle_mesh(cgalmesh.m);
      |            ^
/nix/store/qg5xvm9icynvn84inj4g0z6zq9ibl06b-cgal-5.6.2/include/CGAL/Iterator_range.h:49:5: note: selected 'begin' function with iterator type 'CGAL::Surface_mesh<CGAL::Point_3<CGAL::Epick>>::Index_iterator<CGAL::SM_Vertex_index>'
   49 |   I begin() const
      |     ^
/build/source/src/libslic3r/MeshBoolean.cpp:166:16: error: non-const lvalue reference to type 'reference' (aka 'CGAL::SM_Face_index') cannot bind to a temporary of type 'reference' (aka 'CGAL::SM_Face_index')
  166 |     for (auto &face : cgalmesh.faces()) {
      |                ^    ~
/nix/store/qg5xvm9icynvn84inj4g0z6zq9ibl06b-cgal-5.6.2/include/CGAL/Iterator_range.h:49:5: note: selected 'begin' function with iterator type 'CGAL::Surface_mesh<CGAL::Point_3<CGAL::Epick>>::Index_iterator<CGAL::SM_Face_index>'
   49 |   I begin() const
      |     ^
3 warnings and 2 errors generated.

[schdro]

Isn't a const reference, i.e. adding const, enough on both list traversals (to avoid value copy)?

At least that is what Gentoo Linux applies as compile patch since considerable time, see
https://gitweb.gentoo.org/repo/gentoo.git/tree/media-gfx/superslicer/files/superslicer-2.5.59.8-fix-compilation-error-gnu17.patch

[FliegendeWurst]

That also works, I suppose. In that case I would suggest to apply the Gentoo patch instead of this PR.