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.
