This is a private type, and the CSS default value is "stretch", which
is how we were parsing and printing NONE anyway.
With this fix we can use the enum stuff for printing/parsing
By default, a background image is stretched. Instead, it is worth to
have a tiled background.
This patch allows background surfaces to be repeated or not, and should
be compatible with future extensions and CSS.
https://bugzilla.gnome.org/show_bug.cgi?id=663522
