Commit 6c6c89c63470b5 added a pill around the default avatar, but
assumed the sizes from the vertical widget used on the lock screen.
In order to fix the horizontal widget on the login screen, move the
size-specific bits to the corresponding .horizontal and .vertical
sections, and half the sizes for the former (which corelates with
the icon sizes).
https://gitlab.gnome.org/GNOME/gnome-shell/issues/2242