This makes GdkPixbuf opaque within the gdk-pixbuf sources, and formalizes the distinction between immutable/bytes and mutable/pixels modes by separating them with an enum and some inner structs.
It also makes the loaders not use gdk-pixbuf-private.h - they look like normal gdk-pixbuf client code now.
(Uh, shouldn't we have a "ninja test" stage for CI or something?)