Refining the 8-tap interpolation filters.

Fixes a rounding issue with the 8-tap filters, which allows us to
use sharper filters that before with a little better performance.
Results on derf (0verall gain 0.76):
http://www.corp.google.com/~debargha/vp8_results/enhinterp.html
Results on a 720P set (Overall gain 0.61):
http://www.corp.google.com/~debargha/vp8_results/enhinterp_hd.html

Change-Id: I4cd7bdf3583db974dc5589fa64857bc31ac861fa