Alternate reference frame

This commit re-designs the alternate reference frame generation
process. It employs non-local mean approach to produce more stable
pixel estimation for alternate reference frame. It improves the
compression performance gains:
derf   0.5%
hevcmr 0.8%
stdhd  1.3%
hevchr 1.0%

The encoding time at speed 0 is not affected.

Change-Id: Iaa757f0da189ce93812d69617a81bf630d449848
1 file changed