Includes unit tests for conformance and speed.

SSSE2/SubsampleSpeedTest:
4x4: C time = 868 us, SIMD time = 200 us (~4.3x)
8x8: C time = 3054 us, SIMD time = 293 us (~10x)
16x16: C time = 11887 us, SIMD time = 760 us (~16x)

AVX2/SubsampleSpeedTest:
4x4: C time = 784 us, SIMD time = 205 us (~3.8x)
8x8: C time = 2774 us, SIMD time = 307 us (~9x)
16x16: C time = 10978 us, SIMD time = 489 us (~22x)

Change-Id: I7d5958097542599d57d1a9f9a0a1b809c6a345b0