Originally committed as revision 17281 to svn://svn.ffmpeg.org/ffmpeg/trunk
* It is also used by hardware acceleration APIs as a callback
* to pass specific (partially decoded) data to the calling
* application so it could be passed to the rendering hardware.
+ * In that mode all relevant function paramerers are inside special
+ * structure passed by AVFrame in place of the pixel data.
+ * The exact structure depends on the acceleration API.
+ * The application is allowed to make changes in the strucure.
* - encoding: unused
* - decoding: Set by user.
* @param height the height of the slice