add async+await to loadSprite

src/gamelib.js

@@ -132,8 +132,8 @@ export class Gamelib {
         ev.preventDefault();
     }
 
-    loadSprite(name, width, height) {
-        return new Promise((resolve) => {
+    async loadSprite(name, width, height) {
+        return await new Promise((resolve) => {
             const sprite = new OffscreenCanvas(width, height);
 
             const image = new Image();