add avcodec_get_context_defaults2() / avcodec_alloc_context2() which take CodecType as an additional parameter
also mark them as NOT part of the public API yet, so we can change their argument to CodecID if we decide to do so


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8283 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b