boxlayout: Fix broken min-size-for-opposite-size

boxlayout: Fix broken min-size-for-opposite-size

Assume a vbox with 2 wrapping labels saying
  Hello World
  Hi Ho
being measured for their minimum width for 3 rows of text.
This should be layouted like
  Hello
  World
  Hi Ho
and measured accordingly.

However, previously this was layouted as
  Hello World
  Hi Ho
with 1.5 lines being assigned to both labels.
That will obviously not compute the above wrapping which clearly
results in a smaller min width.

A reftest testing exactly this was included.