The self-guided filter speed tests show that:
- The SSE4.1 implementation of FAST SGR is ~35% faster than the corresponding
  implementation of SGR;
- The AVX2 implementation of FAST SGR is ~28% faster than the corresponding
  implementation of SGR.

Change-Id: Iecdc1f8cee79500084c71d06dbb02d804272aa99

Improvements have been made to calc_ab for both the
SSE4.1 and AVX2 versions of the self-guided filter.
These result in an increase in the speed of between
3% and 5% depending on the bit depth.

Change-Id: I83a12ba452fcbb61cce5066801ae213e23c609cd

The self-guided filter has now been implemented using
the intrinsics for AVX2. The corresponding speed and
correctness tests have also been added.

Note: All AVX2 functions are in synonyms_avx2.h, as
GCC produces 'ABI change' warnings if they are
included in synonyms.h.

Change-Id: I2a283a4acf8c01ee835d5edc526abc242d87ad9b