Improve the speed of the warp filter itself by ~30%. This leads
to an overall decoder speedup of 5-20%, depending on bitrate,
for the global-motion experiment, and a small speedup for
warped-motion.

Applies a very minor change to the rounding during filter
selection (ROUND_POWER_OF_TWO makes slightly more sense here
than ROUND_POWER_OF_TWO_SIGNED, and is faster)

Change-Id: I3f364221d1ec35a8aac0d2c8b0e427f527d12e43