track nesting depth in boxes.

js/world.mjs

@@ -8,6 +8,7 @@ export const CENTER = Math.floor(BOX_SIZE / 2);
 function create_world (size) {
 	return {
 		tiles: Array(size).fill(0).map(_ => Array(size).fill(0)),
+		depth: 0,
 	};
 }
 const world = create_world(BOX_SIZE);
@@ -29,6 +30,7 @@ export function get_tile (world, x, y) {
 export function create_box (parent) {
 	return {
 		...create_world(BOX_SIZE),
+		depth: parent.depth + 1,
 		parent,
 	};
 }