video/filter: Restore the previously removed tfields filter.

I disagree with the conclusion of Issue #19 since afaict none of the libpostproc filters provide deinterlacing which doubles the frame rate to match the field rate of the stream. Yadif is just too CPU hungry for HD content for my machine to keep up with.

Since I will be using the tfields filter myself I might as well send the patch upstream.

Compared to the original version it has been updated with code from yadif to use the real delta when calculating the PTS for extra frames instead of using a hardcoded value.

Please pull!