The various avcodec_thread_init() functions are updated to return
immediately after setting avctx->thread_count.  This allows -threads 0
to pass through to codecs.  It also simplifies the usage for apps
using libavcodec.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@21358 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b