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/cycle/cycle.c

@@ -231,7 +231,7 @@ preview_selected_view(struct view *view)
 	cycle->preview_node = &view->scene_tree->node;
 
 	/* Create a dummy node at the original place of the previewed window */
-	struct wlr_scene_rect *dummy_rect = wlr_scene_rect_create(
+	struct wlr_scene_rect *dummy_rect = lab_wlr_scene_rect_create(
 		cycle->preview_node->parent, 0, 0, (float [4]) {0});
 	wlr_scene_node_place_below(&dummy_rect->node, cycle->preview_node);
 	wlr_scene_node_set_enabled(&dummy_rect->node, false);