Overall change (using dual buffer scheme for superblocks of both inter
and intra modes) reduces speed 2 runtime:
bluesky_1080p at 6000kbps:   263553ms -> 257441ms
riverbed_1080p at 8000kbps:  233230ms -> 225308ms.

Change-Id: Idf8d70f768a4b0d97b2a8506372c57b7b4022119