New feature: Render Layers

Now it's much easier to manage the z-order position when rendering multiple entity outputs. For example:

```ruby
world :previous_behavior do
    # Automatically rendered in ascending entity ID order (the pre-existing behavior)
    entity BackgroundImage
    entity Panel
    entity Button
    entity MouseCursor
    # Previously these entities would be rendered to args.outputs,
    # now this has changed to args.outputs[:default].
end

# Set a named output target (args.outputs[KEY]) for all instances of an entity class
entity :background_image do
    # ...components
    layer :background
end

entity :panel do; layer :ui; end
entity :mouse_cursor do; layer :top; end

world :rendering_layers do
    entity BackgroundImage
    entity Panel
    entity Button, layer: { key: :ui } # Same as above, but specific to this entity
    entity MouseCursor

    # List the layers in the order they should be rendered. Entities of any other layer
    # (except for :default) not included in this list will not be rendered.
    # Within each layer, entities will be rendered in order of ascending id.
    render :background, :ui, :default, :top

    # You can omit the `render` line if you only want to render :default
    # render :default # (implied)
end

world :using_layer_blocks do
    # You can also use a `layer` block to assign a layer to every entity inside
    layer :background do
        entity BackgroundImage
        entity Panel
        entity SplashImage
    end

    # You can declare your layer blocks in any order.
    layer :top do; entity MouseCursor; end
    layer :ui do; entity Button; end

    # Rendered to :default (unless a layer is specified in the entity class)
    entity Image

    # If you omit :default, it will be added at the end of the list.
    render :background, :ui, :top # => [:background, :ui, :top, :default]
end
```