codecs: mpeg2decoder: Creating the field based on its arriving time.

codecs: mpeg2decoder: Creating the field based on its arriving time.

Spec says:
In a frame picture top_field_first being set to ‘1’ indicates that the
top field of the reconstructed frame is the first field output by the
decoding process. top_field_first being set to ‘0’ indicates that the
bottom field of the reconstructed frame is the first field output by
decoding process.

Here, the "output" should be interpreted just as the output order, not
including the decoding order. The field should be decoded as the order
they comes in the stream. Namely, no matter top_field_first is 0 or 1,
the first coming field is the first one to be decoded.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1929>