public void Flatten()
{
var layer1 = new Layer(_image, "test1", ImageType.Rgb);
_image.InsertLayer(layer1, 0);
var layer2 = new Layer(_image, "test2", ImageType.Rgb);
_image.InsertLayer(layer2, 0);
Assert.AreEqual(2, _image.Layers.Count);
_image.Flatten();
Assert.AreEqual(1, _image.Layers.Count);
}