Fix image.decode() return

Author: ahocevar

slides.md

@@ -685,7 +685,7 @@ const source = new Source({
   loader(z, x, y, {signal}) {
     const image = new Image();
     image.src = `${z}/${x}/${y}.png`};
-    return image.decode();
+    return image.decode().then(() => createImageBitmap(image));
   }
 });
 ```
@@ -710,7 +710,7 @@ const source = new Source({
       bbox: extent.join(','),
     });
     image.src = `map?crs=[EPSG:3857]&scale-denominator=1&${query.toString()}`;
-    return {image: image.decode(), pixelRatio: 1};
+    return {image: image.decode().then(() => createImageBitmap(image)), pixelRatio: 1};
   }
 });
 ```