add getter on mesh shared ptr(suggested by @Vojta)

src/libslic3r/Model.hpp

@@ -618,6 +618,7 @@ public:
     void                set_mesh(std::shared_ptr<const TriangleMesh> &mesh) { m_mesh = mesh; }
     void                set_mesh(std::unique_ptr<const TriangleMesh> &&mesh) { m_mesh = std::move(mesh); }
 	void				reset_mesh() { m_mesh = std::make_shared<const TriangleMesh>(); }    
+    const std::shared_ptr<const TriangleMesh>& get_mesh_shared_ptr() const { return m_mesh; }
     // Configuration parameters specific to an object model geometry or a modifier volume, 
     // overriding the global Slic3r settings and the ModelObject settings.
     ModelConfigObject	config;