SSE2/SSSE3 optimizations and unit test for sub_pixel_avg_variance().

Encoding of bus @ 1500kbps (first 50 frames) goes from 3min57 to
3min35, i.e. approximately a 10.5% speedup. Note that the SIMD versions
which use a bilinear filter (x_offset & 7 || y_offset & 7) aren't
perfectly interleaved, and can probably be improved further in the
future. I've marked this with a few TODOs/FIXMEs in the code.

Change-Id: I5c9e900c0f0d32e431a50fecae213b510b2549f9