Adding support for error concealment in multi-threaded decoding

Also includes a couple of error concealment bug fixes:
- the segment_id wasn't properly initialized when missing
- when interpolating and no neighbors are found, set to zero
- clear the qcoef buffer when concealing an MB

Change-Id: Id79c876b41d78b559a2241e9cd0fd2cae6198f49
4 files changed