Support dark/light variant wallpapers introduced in GNOME 42
The format used for wallpapers changed in 42 to use a light and dark variant instead of the mornight/day/night trio. Aditionally in 42 the wallpapers come in jpeg format, while in 43 they come in webp which requires https://github.com/aruiz/webp-pixbuf-loader/blob/mainline/meson_options.txt.