public IRenderableTexture GetExistingTexture([NotNull] string kn5Filename, string textureName) {
if (kn5Filename == null) throw new ArgumentNullException(nameof(kn5Filename));
var key = kn5Filename + "//" + textureName;
IRenderableTexture texture;
return _textures.TryGetValue(key, out texture) ? texture : null;
}