avcodec: Add padding after the remaining AVFrames

This limits ABI issues in case libavcodec is linked to a libavutil with larger AVFrame
Which can happen if they are shiped in seperate binary packages and libavutil is upgraded

A cleaner alternative would be to replace them by pointers but this would likely cause
a small speedloss

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>