SMOOTH_PRED: Use 8-bit weights.

Using 8-bit weights gives similar results as 12-bit, with only noise
level difference. Here's what 8-bit looks like compared to 12-bit:

* AWCY Objective-1-fast:
                          high latency          low latency
ALL keyframes             0.00                  0.01
Video                     0.00                  0.04

* Google sets:

All Keyframes:
lowres: 0
midres: -0.001
hdres: -0.001

Video overall:
lowres: 0
midres: -0.063
hdres: 0.026

Change-Id: Ibed6015aa7cce12fcc6f314ffde76624df4ad2a1