Remove belt cache array; use BeltComponent instead

Removed the belt cache array. Follow-up belts are cached in the belt's
BeltComponent instead. This change also removes the recursive follow-up
search, which could cause a stack overflow for an extremely long belt
chain.

Saves one object allocation per belt per change, two very large array
allocations per change, many function calls, and belts are only visited
exactly once per change.

src/js/game/components/belt.js

@@ -5,6 +5,7 @@ import { BaseItem } from "../base_item";
 import { Vector, enumDirection } from "../../core/vector";
 import { Math_PI, Math_sin, Math_cos } from "../../core/builtins";
 import { globalConfig } from "../../core/config";
+import { Entity } from "../entity";
 
 export class BeltComponent extends Component {
     static getId() {
@@ -12,6 +13,7 @@ export class BeltComponent extends Component {
     }
 
     static getSchema() {
+        // The followUpCache field is not serialized.
         return {
             direction: types.string,
             sortedItems: types.array(types.pair(types.float, types.obj(gItemRegistry))),
@@ -34,6 +36,9 @@ export class BeltComponent extends Component {
 
         /** @type {Array<[number, BaseItem]>} */
         this.sortedItems = [];
+
+        /** @type {Entity} */
+        this.followUpCache = null;
     }
 
     /**