Improve SIMD usage of Oklab conversions #75
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I changed the Oklab conversion code to make better use of SIMD operations. So instead of calculating expensive operations for each channel individually, they are now calculated in parallel using SIMD. This makes encoding BC1/2/3 images with perceptual error metric around 15% faster.
I also want to note that none of the 3 expensive operations I vectorized (all the
*_vecfunctions) are particularly SIMD-friendly. Especiallycbrt_vecis. This function alone currently eats up 25% of the total benchmark time. If different approximations were used, another 15% could likely be gained. However, I haven't spent much time looking for that yet.