popover: Invalidate borders on tail position changes
popover: Invalidate borders on tail position changes

There's nothing doing this explicitly, so tail position changes may
leave trails of older positions.

https://bugzilla.gnome.org/show_bug.cgi?id=756449

alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita