Specified role weights not preserved in admin

Problem/Motivation

Steps to reproduce:

1. Navigate to admin/people/permissions/roles
2. Show row weights
3. Assuming a default Drupal install, with roles anonymous user, authenticated user, administrator, with weights 0, 1, 2, respectively:
4. Add a role of manager (its weight will default to 3)
5. Set manager's weight to 10. Save.
6. The weight field will still show as 3.

Technically, manager's weight was updated to 10 in the role table, but user_admin_roles() doesn't bother to check what the weight was in the table (because it gets its info from user_roles(), which doesn't disclose weight), which seems to be by design as the foreach loop uses a zero-based counter.

I'm using a Features-based workflow where I sometimes provide a role that holds various permissions for the content types, taxonomies, etc. defined by that feature. Since features get turned on on some sites but not others, and the sites may have roles besides ones defined by features, this means that every time I visit the role admin page, those weights get changed and the feature is overridden. This is inconsistent with how weights work elsewhere in Drupal, where gaps are permitted.

Proposed resolution

Changing user_roles()' signature is probably out of the question. Right now I have this in an alter hook:

  $role_weights = db_query('SELECT r.rid, r.weight FROM {role} r')->fetchAllKeyed();

  foreach ($role_weights as $rid => $weight) {
    if (isset($form['roles'][$rid])) {
      $form['roles'][$rid]['#role']->weight = $weight;
      $form['roles'][$rid]['#weight'] = $weight;
      $form['roles'][$rid]['weight']['#default_value'] = $weight;
    }
  }

Doing a second query in user_admin_roles() to get the weight would likely have a negligible performance impact, which shouldn't matter for a page that only admins need to access occasionally.

User interface changes

This should be an invisible change for users who use the drag and drop handles. For users of numeric row weights, this will make the role admin weight behavior consistent with weights elsewhere in Drupal (e.g., field UI, menu, taxonomy).