- For experiment EXT_INTERP under high bit depth.
- Add unit test to verify bit-exact.
- Speed performance improvement:
  On Xeon E5-2680, park_joy_1080p_12.y4m, 50 frames, encoding time
  drops from 6682503 ms to 5390270 ms.

Change-Id: Iea4debf5414f3accf1eb5672abeab56a0539ac77