ivtc: new inverse telecine filter

ivtc: new inverse telecine filter

This filter converts interlaced content that was originally
telecine'd from 24 fps (or similar) progressive content.  It works
approximately like videorate, but with awareness of interlacing.
It merges neighboring fields in the input interlaced stream with
the goal of minimizing combing artifacts, while keeping the output
framerate constant.  If it cannot avoid combing artifacts, it will
reconstruct the image from a single field.  Note that this filter
does not autodetect the framerate, but will automatically fixate
at 24 fps.