background: Add exception to no-loop-func rule

Modifying variables from an outer scope in functions created in a loop
is considered problematic by eslint, because the variable value in the
resulting closure is often not what the coder intended.

In this particular case however, the scoping is correct, so add a comment
to disable the rule locally.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/818

js/ui/background.js

@@ -403,6 +403,7 @@ var Background = GObject.registerClass({
                 if (numPendingImages == 0)
                     finish();
             } else {
+                // eslint-disable-next-line no-loop-func
                 let id = image.connect('loaded', () => {
                     image.disconnect(id);
                     numPendingImages--;