avformat/wtvdec: demux mpeg2 extradata

Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/wtvenc: pad judiciously when writing mpeg2 extradata

Padding rule described here http://msdn.microsoft.com/en-us/library/windows/desktop/dd390707(v=vs.85).aspx

Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>