メッシュの頂点バッファの数を取得します（読み取り専用）。
Most Meshes contain only one vertex buffer, but some (such as skinned Meshes on some platforms)
might contain more than one. This property is mostly useful together with GetNativeVertexBufferPtr
to enable Mesh manipulation from native code plugins.
See Also: Native code plugins, GetNativeVertexBufferPtr.