Added encode/decode recon buffer match test to vpxenc

The commit added a new command line option --test-decode to vpxenc.
The option enables encoder to decode compressed frames and test recon
buffers from the decode against those from encode for mismatch.

There are a few limitations on this option currently, one of them
being the match test is not done on a number of lagged frames at
the end of an encoding.

Change-Id: I80c29b46dcdcea9f48107a506b235743068862fe