Fix erroneous usage of height-for-width apis in gtk_widget_real_adjust_size_allocation().

Fix erroneous usage of height-for-width apis in gtk_widget_real_adjust_size_allocation().

When fitting a widget into its allocation, the second dimension
is always dependent on the first, so gtk_widget_get_preferred_size()
cannot be used directly (because we want the natural height for
the allocated width, not the natural height for the natural width,
which is generally a smaller height than the height-for-minimum-width
or height-for-allocated-width).

Added test to testadjustsize to ensure proper behaviour.