This uses MB variance to change the RDO weight for mode decision
and quantization.
Activity is normalized against the average for the frame, which is
currently tracked using feed-forward statistics.
This could also be used to adjust the quantizer for the entire
frame, but that requires more extensive rate control changes.
This does not yet attempt to adapt the quantizer within the frame,
but the signaling cost means that will likely only be useful at
very high rates.
Change-Id: I26cd7c755cac3ff33cfe0688b1da50b2b87b9c93
8d0f7a01e6