[Processing/x86] Add an SSSE3 implementation of GeneralBilinearFastDownsample

Keep track of relative pixel offsets and utilize pshufb to efficiently extract relevant pixels for horizontal scaling ratios <= 4. Fall back to a generic approach for ratios > 4. Note that the generic approach can be backported to SSE2. The implementation assumes that data beyond the end of each line, before the next line begins, can be dirtied; which AFAICT is safe with the current usage of these routines. Speedup is ~6.67x/~3.26x (32-bit/64-bit) for horizontal ratios <= 2, ~6.24x/~3.00x for ratios within (2, 4], and ~4.89x/~2.17x for ratios > 4 when not memory-bound on Haswell as compared with the current SSE2 implementation.

SASindre Aamås committed 10 years ago