Add Mesh::id() for consistency.

src/Mesh.h

@@ -407,6 +407,14 @@ class MAGNUM_EXPORT Mesh {
         /** @brief Move assignment */
         Mesh& operator=(Mesh&& other) noexcept;
 
+        /**
+         * @brief OpenGL mesh ID
+         *
+         * If @extension{APPLE,vertex_array_object} is not available, returns
+         * 0.
+         */
+        GLuint id() const { return _id; }
+
         /**
          * @brief Index size
          *