Adds support for enhanced interpolation for subpel motion

Adds support for enhanced interpolation for subpel motion
using an 8-tap filter.

The results with 3 different 8-tap filters on the derf set are in:
http://www.corp.google.com/~debargha/vp8_results/enhinterp.html
The one that gives the most gain achieves an overall gain of about
0.6%. The results for a set of 12 hd (720p) videos are in:
http://www.corp.google.com/~debargha/vp8_results/enhinterp_hd.html
with max gain of 0.55% with the same filter. The best filter apparently
achieves the best trade-off between pass band ripple and stop band
attenuation.

Change-Id: I919e28ae245c0493147fa0864f8c9d048a9dd530