picture: make poc the first field in Dav1dPicture

This prevents warnings on some old compilers that don't like structs that start with an array being initialized with { 0 }.

Chromium seems to use one of these compilers alongside -Werror, so what should be a bogus and harmless -Wmissing-braces warning becomes a fatal error.

See http://ffmpeg.org/pipermail/ffmpeg-devel/2018-October/235632.html