graphics/gegl3: unbreak with ffmpeg 4.0

./ff-save.c:293:17: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

c->flags |= CODEC_FLAG_GLOBAL_HEADER;
            ^

./ff-save.c:634:18: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

c->flags |= CODEC_FLAG_GLOBAL_HEADER;
            ^

./ff-save.c:702:30: error: use of undeclared identifier 'AVFMT_RAWPICTURE'

if (!(oc->oformat->flags & AVFMT_RAWPICTURE))
                           ^

./ff-save.c:806:28: error: use of undeclared identifier 'AVFMT_RAWPICTURE'

if (oc->oformat->flags & AVFMT_RAWPICTURE)
                         ^

PR: 227726