wavpack demuxer: export full wavpack blocks.

Currently the demuxer shaves the blocks and exports only the
information that is useful to the decoder.

Exporting the blocks just as they are stored is simpler to understand
and will make remuxing wavpack easier.
matroskadec: export full wavpack blocks.

This allows us to get rid of demuxer-specific hacks in the decoder and
will allow streamcopy from matroska once we have a wavpack muxer.