Quantize (64-bit only, for now) SSSE3 SIMD.
Total encoding time for first 50 frames of bus (speed 0) @ 1500kbps goes 2min34.8 to 2min14.4, i.e. a 10.4% overall speedup. The code is x86-64 only, it needs some minor modifications to be 32bit compatible, because it uses 15 xmm registers, whereas 32bit only has 8. Change-Id: I2df53770c2e850813ffa713e1a91b45b0082b904