Support wlr-layer-shell as a Backend

[bbb651]

I would like cage to have the ability to run as a wlr-layer-shell client (with configuration given through command line arguments) when running it nested inside other compositors. This is different from #95 that is about supporting the wlr-layer-shell protocol as a server.

My use case is running a SDL2 application as a wallpaper. SDL2 doesn't plan on supporting the protocol, there is a old pr that adds support but I don't have the source for my binary and SDL2 is statically linked.

I think this will require upstream support in wlroots as it doesn't like this exists in the backends.

[emersion]

It's possible to implement this without wlroots changes, see e.g. https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/master/examples/embedded.c?ref_type=heads