Adding error-concealment to the decoder.

The error-concealer is plugged in after any motion vectors have been
decoded. It tries to estimate any missing motion vectors from the
motion vectors of the previous frame. Intra blocks with missing
residual are replaced with inter blocks with estimated motion vectors.

This feature was developed in a separate sandbox
(sandbox/holmer/error-concealment).

Change-Id: I5c8917b031078d79dbafd90f6006680e84a23412