wallpaper plugin: solid fill/gradients broken

As reported in Debian on http://bugs.debian.org/917379 :

“ following a recent update the wallpaper plugin ceased to render solid fills and horizontal/vertical gradients.

If I put in the backgrounds list an image it gets rendered irrispective of the image's position in the list, otherwise the default background image (the one instantiated by the login manager) is displayed instead of my solid colors/gradients. ”