Use new H264Ref.reference field to track field picture flags. The
H264Picture.reference flag in DPB is now irrelevant here.

This is a regression from git commit a12d3188, and that affected
multiple interlaced video streams.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>