We want to center the subpicture region, but the text itself needs
to be rendered left justified in order to get consistent placement
of captions (e.g. indentation).

Introduce a new alignment type which centers the subpicture, but
not the text it contains within (and enable it for the EIA-608 decoder).
Signed-off-by: Rafaël Carré <funman@videolan.org>