scanner: methods cannot have an out-arg as their first arg

This ensures that if the first argument of a function like

  gboolean gdk_rgba_parse (GdkRGBA *rgba, const gchar *spec);

is annotated as being an out-arg, the result is a class function with two
arguments, not a method with one argument.  Previously, the (out) annotation
was simply ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=682124