Remove layering in favor of z-ordering

[simbleau]

Rather than forcing the developer to use our layers, e.g.

commands
        .spawn(VelloVectorBundle {
            layer: bevy_vello::Layer::Background,
            ..
        })

Wouldn't it make sense to pass in a "z-ordering" or something universally useful?

We used layer as a quick and dirty painter's algorithm implementation but it would be better to use a "z-ordering"

[simbleau]

This was recently implemented