`Skybox` `image` is now optional

The image field of the Skybox component now has the type Option<Handle<Image>> instead of Handle<Image>. A Skybox component without an image will not draw anything, just as though it isn't present.

If you were creating a skybox with an image, wrap the image handle in Some:

// 0.18
Skybox {
    image: my_skybox,
    brightness: 1000.0,
    ..default()
}

// 0.19
Skybox {
    image: Some(my_skybox),
    brightness: 1000.0,
    ..default()
}

If you were previously creating a Skybox component with a placeholder image to be changed later, you can now remove the placeholder:

// 0.18
Skybox {
    image: cubemap_image_that_will_not_actually_be_seen,
    brightness: 1000.0,
    ..default()
}

// 0.19
Skybox {
    brightness: 1000.0,
    ..default()
}