Authentication modal dialog: the spinner should disappear when loading is done.

Also, when using spinners as background images, the re-painted area should be the
smallest possible one. See similar performance issue in #33311.

Fixes #34951.
Built from https://develop.svn.wordpress.org/trunk@35925


git-svn-id: https://core.svn.wordpress.org/trunk@35889 1a063a9b-81f0-0310-95a4-ce76da25c4cd