This is not strictly an API change as GdkDrawable is typedeffed to
GdkWindow, but it changes the header, so I'm marking it as such.
gdk_cairo_create() can only be used with windows these days, so it makes
sense to pass a window. With that, we can alseo remove the
set_cairo_clip() vfunc from GdkDrawable and implement it inside
gdkwindow.c.