- Apply 8-pixel vertical filtering direction parallelism.
- Add unit tests to verify bit exact.
- Encoder speed improves ~29% (enable EXT_INTERP) on Xeon E5-2680.
- Combinational cycle count of vp10_convolve() drops from 26.06%
  to 6.73%.

Change-Id: Ic1ae48f8fb1909991577947a8c00d07832737e57