tree-wide: use die_if_null() for wlr_scene alloc failures

wlr_scene_*_create() functions all allocate memory via calloc() and
return NULL if the allocation fails. Previously, the failures were
handled in any of 3 different ways:

 - sending a wayland protocol error
 - exiting labwc with an error
 - segfault (no NULL check at all)

Since labwc does not attempt to survive heap exhaustion in other
allocation paths (such as `znew`), it seems more consistent to use the
same die_if_null() check used in those paths to exit with an error.

For the three most common create() functions (tree, rect, buffer),
add small lab_wlr_ wrappers to common/scene-helpers.

src/xwayland-unmanaged.c

@@ -68,9 +68,11 @@ handle_map(struct wl_listener *listener, void *data)
 		seat_focus_surface(&unmanaged->server->seat, xsurface->surface);
 	}
 
-	unmanaged->node = &wlr_scene_surface_create(
-			unmanaged->server->unmanaged_tree,
-			xsurface->surface)->buffer->node;
+	struct wlr_scene_surface *scene_surface = wlr_scene_surface_create(
+		unmanaged->server->unmanaged_tree, xsurface->surface);
+	die_if_null(scene_surface);
+	unmanaged->node = &scene_surface->buffer->node;
+
 	wlr_scene_node_set_position(unmanaged->node, xsurface->x, xsurface->y);
 	cursor_update_focus(unmanaged->server);
 }