ClutterImage as is, is very basic and doesn't really serve any purposes
other than being inherited by StImageContent. As we cannot easily push
ImageContent to Mutter by merging it with ClutterImage because it will
bring more GdkPixbuf usages and also because Clutter doesn't really deal
with theming bits.
Instead, merge it with StImageContent and let GNOME Shell take over from
here.