clutter/box-layout: Add spacing when computing opposite orientation size

ClutterBoxLayout calculates the preferred size of the opposite
orientation (so for example the height if the orientation is horizontal)
by getting the preferred size of the real orientation first, and then
the preferred size of the opposite orientation, using the other size as
for_width/height when doing the request.

Right now, for non-homogeneous layouts this for_width/height does not
adjust for the spacing set on the box layout. This leads to children
being passed a slightly larger for_width/height, which in case of
ClutterText might cause the line to not wrap when it actually should.
This in turn means we can end up with an incorrect preferred size for
the opposite orientation, leading to a wrong allocation.

So fix that and adjust for the spacing just as we do for homogeneous
layouts by subtracting the total spacing from the available size that is
distributed between children.

This fixes the wrong height of the checkbox label reported in
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2574.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1333

clutter/clutter/clutter-box-layout.c

@@ -315,6 +315,8 @@ get_preferred_size_for_opposite_orientation (ClutterBoxLayout   *self,
     }
   else
     {
+      size -= (nvis_children - 1) * priv->spacing;
+
       /* Bring children up to size first */
       if (isnormal (size) || size == 0)
         {