fecdec: Wait until the second FEC group is complete.

Currently the fecdec filter starts writing output as soon as the first slice of the
second group arrives. This makes the logic in fecdec_filter.c quite convoluted
and does not help much to avoid buffer underruns.

Waiting for completion of the second group simplifies the code quite a bit and
adds more more buffering to the fecdec filter which should make buffer underruns
less likely.