ff_get_plane_bytewidth().
The new implementation is more generic, more compact and more correct.
Originally committed as revision 24786 to svn://svn.ffmpeg.org/ffmpeg/trunk
libavcodec/imgconvert.c and make them public in libavcore/imgutils.h,
with the names av_fill_image_linesizes() and av_fill_image_pointers().
Originally committed as revision 24583 to svn://svn.ffmpeg.org/ffmpeg/trunk