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/xdg-popup.c

@@ -164,6 +164,8 @@ xdg_popup_create(struct view *view, struct wlr_xdg_popup *wlr_popup)
 	}
 	wlr_popup->base->surface->data =
 		wlr_scene_xdg_surface_create(parent_tree, wlr_popup->base);
+	die_if_null(wlr_popup->base->surface->data);
+
 	node_descriptor_create(wlr_popup->base->surface->data,
 		LAB_NODE_XDG_POPUP, view, /*data*/ NULL);
 }