Everything can now be shallow-copied again

Makes shallow copy of decoder possible again

Avoids conflict with LPCNet, so that LPCNet headers can be included in
Opus

progressive decode, avoid storing DRED decoder state

Now generating the extension in place once all the data is already
in the right place.

Adding a 'D' byte to signal the DRED experiment, along with a version
number byte. This entire commit will be reverted once DRED is finalized
and given a non-experimental extension number.

Fixes "FEC buffer full" issue

Properly re-initialize DRED (only) when needed

Probably still has many bugs