Fix build with CGAL 5.6 (again) (SoftFever#6439)

* Fix build with CGAL 5.6

Surface_mesh iterators no longer return references, so it's necessary to
use const references or copies when iterating.

This was previously merged in SoftFever#3045 but isn't references in the
current main branch anymore. No idea where why it is gone, but the issue
still persists thus the patch should still be added.

src/libslic3r/MeshBoolean.cpp

@@ -201,7 +201,6 @@ indexed_triangle_set cgal_to_indexed_triangle_set(const _Mesh &cgalmesh)
     const auto &vertices = cgalmesh.vertices();
     int vsize = int(vertices.size());
 
-    // HvdR: https://github.com/SoftFever/OrcaSlicer/pull/6439 
     for (const auto &vi : vertices) {
         auto &v = cgalmesh.point(vi); // Don't ask...
         its.vertices.emplace_back(to_vec3f(v));