Add image lazy loading

extra/shell.html

@@ -8,6 +8,7 @@
     <script src="js/localforage.min.js"></script>
     <script src="js/drive.js"></script>
     <script src="gameasync/mapping.js"></script>
+    <script src="gameasync/bitmap-map.js"></script>
 
     <title>MKXP</title>
     <style>
@@ -292,6 +293,7 @@
       var hideTimer = 0;
       function getLazyAsset(url, filename, callback) {
           const xhr = new XMLHttpRequest();
+          xhr.responseType = "arraybuffer";
           const pdiv = document.getElementById("progress");
           let showTimer = 0;
           let abortTimer = 0;
@@ -308,10 +310,9 @@
                       pdiv.style.opacity = '0';
                       hideTimer = 0;
                   }, 500);
-                  callback();
-
                   clearTimeout(showTimer);
                   clearTimeout(abortTimer);
+                  callback(xhr.response);
               }
           }
           xhr.onprogress = function (event) {