Add a timeout option to AssetManager.preload()

demos/hello-babylon/demo.js

             '../assets/cat.glb',
             '../assets/magic-circle.png',
             '../assets/it-works.png',
-        ]);
+        ], { timeout: 20 });
     }
 
     /**

plugins/extras/asset-manager.js

     /**
      * Preload one or more assets
      * @param {string|string[]} url URL(s) of the asset(s)
+     * @param {object} [options]
+     * @param {number} [options.timeout] timeout, in seconds
      * @returns {Promise<void>}
      */
-    preload(url)
+    preload(url, options = {})
     {
         if(Array.isArray(url))
-            return Promise.all(url.map(url => this.preload(url)));
+            return Promise.all(url.map(url => this.preload(url, options)));
 
         if(typeof url != 'string')
             return Promise.reject(new TypeError());
         if(this._assetMap.has(filename))
             return Promise.resolve();
 
-        return new Promise(resolve => {
+        return new Promise((resolve, reject) => {
+            const seconds = options.timeout !== undefined ? options.timeout : Infinity;
+            const timeoutFn = () => reject(new Error(`Can't preload assets: timeout!`));
+            const timeoutId = isFinite(seconds) ? setTimeout(timeoutFn, seconds * 1000) : undefined;
+
             fetch(url)
             .then(response => {
                 if(!response.ok)
             .catch(error => {
                 console.warn(`Can't preload asset "${filename}"! ${error.message} (${url})`);
             })
-            .finally(resolve);
+            .finally(() => {
+                clearTimeout(timeoutId);
+                resolve();
+            });
         });
     }