Write SSSE3 sub-pixel filter function

1. Process 16 pixels at one time instead of 8.
2. Add check for both xoffset =0 and yoffset=0, which happens
   during motion search.
This change gave encoder 1%~3% performance gain.

Change-Id: Idaa39506b48f4f8b2fbbeb45aae8226fa32afb3e