backend: return wlr_session in wlr_backend_autocreate() call

Up until now, wlr_backend_autocreate() created the wlr_session and
then stuffed it into struct wlr_multi_backend so that compositors
can grab it later.

This is an abuse of wlr_multi_backend and the wlr_backend API:
wlr_backend_get_session() and wlr_multi_backend.session only exist
to accomodate the needs of wlr_backend_autocreate(). What's more,
the DRM and libinput backends don't implement
wlr_backend_impl.get_session.

Instead, return the struct wlr_session to the compositor in the
wlr_backend_autocreate() call. wlr_backend_get_session() will be
removed in the next commit.

examples/scene-graph.c

@@ -157,7 +157,7 @@ int main(int argc, char *argv[]) {
 	struct server server = {0};
 	server.surface_offset = 0;
 	server.display = wl_display_create();
-	server.backend = wlr_backend_autocreate(server.display);
+	server.backend = wlr_backend_autocreate(server.display, NULL);
 	server.scene = wlr_scene_create();
 
 	server.renderer = wlr_renderer_autocreate(server.backend);