Set the "layer" on which the surface appears (controls if it is over top of or below other surfaces).
The layer may be changed on-the-fly in the current version of the layer shell protocol, but on compositors that only support an older
version the window is remapped so the change can take effect.
Default is gtk_layer_shell_layer_top
| window |
A layer surface. |
| layer |
The layer on which this surface appears. |