backend/session: add noop session

This is the first step towards being able to run via DRM leasing and on render
nodes.

Test with:

    export WLR_BACKENDS=drm
    export WLR_SESSION=noop
    export WLR_DRM_DEVICES=/dev/dri/renderD128

backend/meson.build

@@ -23,6 +23,7 @@ backend_files = files(
 	'noop/backend.c',
 	'noop/output.c',
 	'session/direct-ipc.c',
+	'session/noop.c',
 	'session/session.c',
 	'wayland/backend.c',
 	'wayland/output.c',