Commit 9b07ce1d0d changed the OSD window's level bar to be a regular
actor instead of a custom drawn bar. The bar actor's width depends on
both the configured level (e.g. 40%) and the available width, however
the width is currently only updated when the configured level changes.
Fix that by properly considering changes to the parent's width as well.
https://bugzilla.gnome.org/show_bug.cgi?id=768317
775187b2e4