Let H.264 decoder skip deblocking across slices

Let H.264 decoder skip deblocking across slices
if running with multiple threads and CODEC_FLAGS2_FAST is set.
Thus, it may decode the slices in parallel to gain speed.

Patch by Andreas Öman: [andreas olebyn nu]

Originally committed as revision 10431 to svn://svn.ffmpeg.org/ffmpeg/trunk