These buffers are just a way to store frame pointers and be able to
modify them without touching the original ones.

The two dependent decoders (WMV2 and VC1) do not need special care for
these fields: the former does not seem to use the dest buffers, while
the latter reinits them every time to the current frame data buffers.

So only keep a local copy rather than the one from mpegvideo.