avcodec_get_chroma_sub_sample(), directly access
av_pix_fmt_descriptors instead.
Remove some of the dependancies of lavfi on lavc.
Originally committed as revision 21575 to svn://svn.ffmpeg.org/ffmpeg/trunk
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