fix: fix building on C++20 - fix removal of shared_ptr.unique

[aminya]

This replaces the removed shared_ptr.unique with .use_count() == 1
See this for the explanation
https:// en.cppreference.com/w/cpp/memory/shared_ptr/unique

Cherry-picked from #28

• It fixes building on C++ 20 by replacing the removed method shraed_ptr.unique with shared_ptr.use_count == 1
  https://en.cppreference.com/w/cpp/memory/shared_ptr/unique

C:/opennurbs/opennurbs_mesh.cpp:14834:34: error: no member named 'unique' in 'std::shared_ptr<ON_Mesh>'
    if (false == item->m_mesh_sp.unique())
                 ~~~~~~~~~~~~~~~ ^

[sbaer]

This is really hard to read. I would recommend changing this to
if (item->m_mesh_sp.use_count() != 1)

[aminya]

This is really hard to read. I would recommend changing this to if (item->m_mesh_sp.use_count() != 1)

Fixed in 71e81b4