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/fullscreen-shell.c

@@ -205,7 +205,7 @@ int main(int argc, char *argv[]) {
 
 	struct fullscreen_server server = {0};
 	server.wl_display = wl_display_create();
-	server.backend = wlr_backend_autocreate(server.wl_display);
+	server.backend = wlr_backend_autocreate(server.wl_display, NULL);
 	server.renderer = wlr_renderer_autocreate(server.backend);
 	wlr_renderer_init_wl_display(server.renderer, server.wl_display);
 	server.allocator = wlr_allocator_autocreate(server.backend,

examples/multi-pointer.c

@@ -272,7 +272,7 @@ int main(int argc, char *argv[]) {
 		.clear_color = { 0.25f, 0.25f, 0.25f, 1 },
 		.display = display,
 	};
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/output-layout.c

@@ -268,7 +268,7 @@ int main(int argc, char *argv[]) {
 	state.layout = wlr_output_layout_create();
 	clock_gettime(CLOCK_MONOTONIC, &state.ts_last);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/pointer.c

@@ -333,7 +333,7 @@ int main(int argc, char *argv[]) {
 		.display = display
 	};
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/quads.c

@@ -189,7 +189,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.outputs);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/rotation.c

@@ -239,7 +239,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.outputs);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

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);

examples/simple.c

@@ -172,7 +172,7 @@ int main(void) {
 		.last_frame = { 0 },
 		.display = display
 	};
-	struct wlr_backend *backend = wlr_backend_autocreate(display);
+	struct wlr_backend *backend = wlr_backend_autocreate(display, NULL);
 	if (!backend) {
 		exit(1);
 	}

examples/tablet.c

@@ -355,7 +355,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.tablet_pads);
 	wl_list_init(&state.tablet_tools);
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/touch.c

@@ -248,7 +248,7 @@ int main(int argc, char *argv[]) {
 	wl_list_init(&state.touch_points);
 	wl_list_init(&state.touch);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}