Horizontal GtkBoxLayout has (unnecessarily) higher minimum height after baseline patches

This has been found by @sthursfield's QA tests for nautilus, in which the headerbar had misteriously grown 2 pixels taller.

But it can be reproduced with gtk4-widget-factory (page 3) or anywhere else where an horizontal GtkBox contains 2 children with different baselines.

Note how, in the second picture (after setting :visible to FALSE on the first entry using the inspector), the box is 2 pixels shorter (36px vs 34px).

What's happening is that the two child entries have different baselines (at 18 vs at 20), as can be seen using the inspector to set :valign on both to BASELINE_CENTER.

Git bisects to e1875876

Reading gtk_box_layout_compute_opposite_size(), I understand it sets a minimum height that's large enough to align baselines... even if no child is asking for baseline alignment.

Both in nautilus and gtk4-widget-factory the children's :valign are simply defaulting to FILL, so they get 2px taller than their minimum height.

The GtkBox minimum height being 2 pixels taller than its children's minimum height is a behavior change.