pixfmt: Add ARIB STD-B76 color transfer characteristic

Adding hybrid log-gamma (https://en.wikipedia.org/wiki/Hybrid_Log-Gamma)
based on the standardization in ARIB STD-B67:
http://www.arib.or.jp/english/html/overview/doc/2-STD-B67v1_0.pdf

The choice of enum value of 18 is consistent with HEVC:
http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=10481

And also with latest proposal for color format in mkv:
https://mailarchive.ietf.org/arch/search/?email_list=cellar&gbt=1&q=Colour+Format+proposal



Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>