The right background isn't drawn in the Greeter session

When we didn’t have gnome-flashback, an ugly all-grey background was displayed. Now that we have gnome-flashback, the default Debian background is displayed. This is an improvement, as at least something is managing the background. We now have to find out why the way we have to set our preferred one in config/chroot_local-patches/gdm-tails-greeter.patch doesn’t work.

Parent Task: #7561 (closed)

Original created by @intrigeri on 7990 (Redmine)