layout: Add support for align-content: stretch (servo#32906)

This adds support for `align-content: stretch` by splitting flex line
layout into two phases. The first phase takes place before determing how
much extra space to allocate for stretching items. Then line layout
finishes, which might cause two layouts for items with `align-self:
stretch`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

Author: 2 people