adding AVVideoFrame
 moving quality, pict_type, key_frame, qscale_table, ... to AVVideoFrame
 removing obsolete variables in AVCodecContext
 skiping of MBs in b frames
 correctly initalizing AVCodecContext
 picture buffer cleanup


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@1302 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b