we'd memset() the codec context to zero, thereby setting audio input to U8
and video to YUV420P. For most video encoders, that actually works, but for
most audio codecs, it doesn't. This patch changes defaults to those set by
avcodec_context_get_defaults() and have ffmpeg figure out the optimal encoding
format itself if not set explicitely (as it does for the non-ffserver-cases
also).

Originally committed as revision 22751 to svn://svn.ffmpeg.org/ffmpeg/trunk