Remove the src attribute from Image objects used with natively supported JPEG images, when pages are cleaned-up/destroyed

This will further help reduce the amount of image data that's currently being held alive, by explicitly removing the `src` attribute.

Please note that this is mostly relevant for browsers which do not support `URL.createObjectURL`, or where `disableCreateObjectURL` was manually set by the user, since `blob:` URLs will be revoked (see the previous patch).
However, using `about:memory` (in Firefox) it does seem that this may also be generally helpful, given that calling `URL.revokeObjectURL` won't invalidate the image data itself (as far as I can tell).

src/display/display_utils.js

@@ -488,6 +488,7 @@ function releaseImageResources(img) {
       URL.revokeObjectURL) {
     URL.revokeObjectURL(url);
   }
+  img.removeAttribute('src');
 }
 
 export {