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/common/scene-helpers.c

@@ -5,6 +5,7 @@
 #include <wlr/types/wlr_output.h>
 #include <wlr/types/wlr_scene.h>
 #include <wlr/util/log.h>
+#include "common/mem.h"
 #include "magnifier.h"
 #include "output.h"
 
@@ -24,6 +25,34 @@ lab_wlr_surface_from_node(struct wlr_scene_node *node)
 	return NULL;
 }
 
+struct wlr_scene_tree *
+lab_wlr_scene_tree_create(struct wlr_scene_tree *parent)
+{
+	struct wlr_scene_tree *tree = wlr_scene_tree_create(parent);
+	die_if_null(tree);
+	return tree;
+}
+
+struct wlr_scene_rect *
+lab_wlr_scene_rect_create(struct wlr_scene_tree *parent,
+		int width, int height, const float color[static 4])
+{
+	struct wlr_scene_rect *rect =
+		wlr_scene_rect_create(parent, width, height, color);
+	die_if_null(rect);
+	return rect;
+}
+
+struct wlr_scene_buffer *
+lab_wlr_scene_buffer_create(struct wlr_scene_tree *parent,
+		struct wlr_buffer *buffer)
+{
+	struct wlr_scene_buffer *scene_buffer =
+		wlr_scene_buffer_create(parent, buffer);
+	die_if_null(scene_buffer);
+	return scene_buffer;
+}
+
 struct wlr_scene_node *
 lab_wlr_scene_get_prev_node(struct wlr_scene_node *node)
 {