Add slice-based parallel H.264 decoding

Add slice-based parallel H.264 decoding
Patch by Andreas Öman % andreas A olebyn P nu %
NB: depends on having a thread library activated at config time, and on
having a source encoded with multiple slices
Original threads:
date: May 18, 2007 11:00 PM
subject: [FFmpeg-devel] Parallelized h264 proof-of-concept
date: Jun 15, 2007 10:10 PM
subject: [FFmpeg-devel] [PATCH] h264 parallelized, (was: Parallelized h264 proof-of-concept)
date: Jun 25, 2007 7:02 PM
subject: Re: [FFmpeg-devel] [PATCH] h264 parallelized

Originally committed as revision 10407 to svn://svn.ffmpeg.org/ffmpeg/trunk