examples: add output-layers example

This new example demonstrates how to use the wlr_output_layer API. It's
a compositor that displays all client surfaces using wlr_output_layer.
To test, one can for instance run:

    build/examples/output-layers -s 'weston-simple-dmabuf-egl & weston-simple-egl'

Under the Wayland backend (where layers work as long as clients use
DMA-BUFs), this should display two surfaces with remote wl_subsurfaces.

examples/meson.build

@@ -55,6 +55,10 @@ compositors = {
 		'src': 'scene-graph.c',
 		'proto': ['xdg-shell'],
 	},
+	'output-layers': {
+		'src': 'output-layers.c',
+		'proto': ['xdg-shell'],
+	},
 }
 
 clients = {