If a subclass (say a child of GtkButton) overrides the non-baseline
size request methods we need to call these, rather than the new
get_height_and_baseline_for_width method.

In order to handle this we make the default for this method to be
NULL, and instead check at runtime which method to call. If any
non-baseline vfunc has changed in a class but the baseline one
hasn't, then we can't use the baseline one.