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
draw_slice().
The chroma values to use are supposed to be those of the input format,
not that of the output format, the latter case was causing crashes and
chroma issues.
Originally committed as revision 20660 to svn://svn.ffmpeg.org/ffmpeg/trunk
