[FIX] web_editor: relative URL in background snippet

When the backround is set with the media modal, directly get the URL of
the IMG element (equivalent to `.getAttribute('src')`) instead of doing
`.src` which returns absolute URI.

This fix uses jQuery wrapper so it still works if called with object:

  $({src: 'test.jpg'}).attr('src') === 'test.jpg'

note: before 17237ff we already used `.attr('src')`.

opw-2215103
closes odoo#47797

Signed-off-by: Nicolas Lempereur (nle) <[email protected]>

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -1019,7 +1019,7 @@ registry.background = SnippetOption.extend({
      * @param {Object} data
      */
     _onSaveMediaDialog: function (data) {
-        this._setCustomBackground(data.src);
+        this._setCustomBackground($(data).attr('src'));
     },
 });