The warp filter for the (0,1) case is changed to use a real
8-tap filter.

Improves coding efficiency.

BDRATE on lowres:
-0.772% (up from -0.633%) with --enable-global-motion
-1.124% (up from -1.001%) with --enable-warped-motion

Change-Id: I296efe36dbc72a7af74773b71b445f19a2aa7205