layout: Ensure that <caption>'s support position: relative
This change adds support for position: relative to table <caption>.
In addition to adjusting their position according to inset values, table
captions must also establish containing blocks for descendants that are
absolutely positioned.
Co-authored-by: Oriol Brufau obrufau@igalia.com
Signed-off-by: Martin Robinson mrobinson@igalia.com