SMOOTH_PRED: Use 12-bit multiplications instead of 18-bit.

Compression performance is roughly neutral:

AWCY:
-----
                 High Latency     Low Latency
  All Keyframes  0.00             0.00
  Video overall  0.01            -0.01

Google sets:
------------

- All Keyframes:

  lowres  -0.001
  midres   0.000
  hdres    0.001

- Video overall:
  lowres   0.019
  midres   0.000
  hdres   -0.013

Change-Id: I89be2739203bf3e2848e4ba7ae2988c625f54513
2 files changed