Overlay layer surfaces render under fullscreen windows

[Ottatop]

I recently changed the order of rendering to, from top to bottom,

1. All windows down to the bottom-most fullscreen window
2. Overlay and top layer shell surfaces
3. The rest of the windows
4. Bottom and background layer shell surfaces

What I failed to realize is that now mako notifications don't show up with a fullscreen window and there is absolutely no way to make them.

A fix is to change the rendering order to

1. Overlay layers
2. Windows down to the bottom-most fullscreen
3. Top layers
4. Rest of the windows
5. Bottom and background layers

to allow that possibility.

[Ottatop]

Fixed in 4a1a01a