Added an AVX2 version of the Wiener filter, along with associated tests. Speed
tests have been added for all implementations of the Wiener filter.

Speed Test results
==================

GCC
---

Low bit-depth filter:
- SSE2 vs C: SSE2 takes ~92% less time
- AVX2 vs C: AVX2 takes ~96% less time
- SSE2 vs AVX2: AVX2 takes ~43% less time (~74% faster)

High bit-depth filter:
- SSSE3 vs C: SSSE3 takes ~92% less time
- AVX2  vs C: AVX2  takes ~96% less time
- SSSE3 vs AVX2: AVX2 takes ~46% less time (~84% faster)

CLANG
-----

Low bit-depth filter:
- SSE2 vs C: SSE2 takes ~84% less time
- AVX2 vs C: AVX2 takes ~88% less time
- SSE2 vs AVX2: AVX2 takes ~27% less time (~36% faster)

High bit-depth filter:
- SSSE3 vs C: SSSE3 takes ~85% less time
- AVX2  vs C: AVX2  takes ~89% less time
- SSS3  vs AVX2: AVX2 takes ~24% less time (~31% faster)

Change-Id: Ide22d7c09c0be61483e9682caf17a39438e4a208