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.c

@@ -13,6 +13,7 @@
 #include "common/array.h"
 #include "common/macros.h"
 #include "common/mem.h"
+#include "common/scene-helpers.h"
 #include "config/rcxml.h"
 #include "config/session.h"
 #include "foreign-toplevel/foreign.h"
@@ -850,11 +851,7 @@ handle_map(struct wl_listener *listener, void *data)
 	if (!view->content_tree) {
 		view->content_tree = wlr_scene_subsurface_tree_create(
 			view->scene_tree, view->surface);
-		if (!view->content_tree) {
-			/* TODO: might need further clean up */
-			wl_resource_post_no_memory(view->surface->resource);
-			return;
-		}
+		die_if_null(view->content_tree);
 	}
 
 	wlr_scene_node_set_enabled(&view->content_tree->node, !view->shaded);
@@ -1058,7 +1055,7 @@ xwayland_view_create(struct server *server,
 	xsurface->data = view;
 
 	view->workspace = server->workspaces.current;
-	view->scene_tree = wlr_scene_tree_create(
+	view->scene_tree = lab_wlr_scene_tree_create(
 		view->workspace->view_trees[VIEW_LAYER_NORMAL]);
 	node_descriptor_create(&view->scene_tree->node,
 		LAB_NODE_VIEW, view, /*data*/ NULL);