add multi slice support

add multi slice support
for main profile H.264 streams. I have tested this on all H264
conformance streams and comparing the result with the current CVS
version, there is 8 streams which decode correctly more frames and 2
streams which are now completely correct with my patch.

This patch also correct some typo in comments.

patch by (Loic Le Loarer <lll+ffmpeg >at< m4x org>)

Originally committed as revision 3758 to svn://svn.ffmpeg.org/ffmpeg/trunk