libavformat/mxfenc.c: Add 'Presentation Y offset' metadata

Previously unset, and some software mishandles files if it is absent

Signed-off-by: Tim Nicholson <tim.nicholson@bbc.co.uk>
Reviewed-by: tomas.hardin@codemill.se
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
