video: confusing top/bottom field flags

These flags are documented as:

 * @GST_VIDEO_BUFFER_FLAG_TOP_FIELD:   The video frame has the top field only. This is the
 *                                     same as GST_VIDEO_BUFFER_FLAG_TFF |
 *                                     GST_VIDEO_BUFFER_FLAG_ONEFIELD (Since: 1.16).
 * @GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD: The video frame has the bottom field only. This is
 *                                     the same as GST_VIDEO_BUFFER_FLAG_ONEFIELD
 *                                     (GST_VIDEO_BUFFER_FLAG_TFF flag unset) (Since: 1.16).

So they should be mutually exclusive. But checking for them is quite confusing, see this simple test:

GST_START_TEST (test_video_flags)
{
  GstBuffer *buf;

  buf = gst_buffer_new ();
  GST_BUFFER_FLAG_SET (buf, GST_VIDEO_BUFFER_FLAG_TOP_FIELD);
  g_assert (GST_BUFFER_FLAG_IS_SET (buf, GST_VIDEO_BUFFER_FLAG_TOP_FIELD));
  g_assert (!GST_BUFFER_FLAG_IS_SET (buf, GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD)); // fail
  gst_buffer_unref (buf);

  buf = gst_buffer_new ();
  GST_BUFFER_FLAG_SET (buf, GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD);
  g_assert (!GST_BUFFER_FLAG_IS_SET (buf, GST_VIDEO_BUFFER_FLAG_TOP_FIELD)); // fail
  g_assert (GST_BUFFER_FLAG_IS_SET (buf, GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD));
  gst_buffer_unref (buf);
}

Because of the way they are defined it's basically useless to check for their presence or not which is pretty confusing.

  GST_VIDEO_BUFFER_FLAG_TOP_FIELD   = GST_VIDEO_BUFFER_FLAG_TFF |
                                      GST_VIDEO_BUFFER_FLAG_ONEFIELD,
  GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD = GST_VIDEO_BUFFER_FLAG_ONEFIELD,

We have the same problem with the equivalent frame flags: GST_VIDEO_FRAME_FLAG_TOP_FIELD and GST_VIDEO_FRAME_FLAG_BOTTOM_FIELD.

Not sure what's the right way to address this. Do we have other flags defined as a combination of other flags?

cc @ndufresne