Use 255 instead of 256, to restrict to 8-bits.

Only noise level differences in performance.

AWCY:
                   High Latency     Low Latency
  All Keyframes    -0.01            -0.01
  Video overall    -0.01            -0.07

Google Set:
                  All KF            Video
lowres            -0.005            -0.029
midres            -0.008             0.028
hdres             -0.010            -0.022      

Note: By moving from 18-bit to 8-bit and then
cutting off at 255 (this change, overall effect is
noise level too (neutral or better).

Change-Id: I9f2852023015e36c01203bafe486ec400b2ba46f