Avoid the need to implement slice direction detection code, thus
reducing code duplication.
See the thread:
"[FFmpeg-devel] [PATCH] Add a slice_dir parameter to avfilter_start_frame()".
Originally committed as revision 20734 to svn://svn.ffmpeg.org/ffmpeg/trunk
"video format and noformat filters" to
"format and noformat video filters",
as suggested by Diego.
Originally committed as revision 20442 to svn://svn.ffmpeg.org/ffmpeg/trunk