V4L2_FBUF_FLAG_OVERLAY
fb->flags |= V4L2_FBUF_FLAG_OVERLAY;
yi->track_osd = (fb->flags & V4L2_FBUF_FLAG_OVERLAY) != 0;
vout->fbuf.flags = V4L2_FBUF_FLAG_OVERLAY;
a->flags = V4L2_FBUF_FLAG_OVERLAY | dev->fbuf_out_flags;