This stuff is pretty hairy since the specifications don't give any
guidance on which widths to use when calculating the intrinsic height of
flex items in a column layout.
However, our old behavior of "treat anything indefinite as fit-content"
was definitely not good enough, so this patch improves the situation by
considering values like `min-content`, `max-content` and `fit-content`
separately from `auto`, and making the whole flex layout pipeline aware
of them (in the cross axis context).
