Add AVX2 conversions from u8 Y', Y'A, R'G'B, and R'G'B'A to float
Y, YA, RGB, and RGBA, respectively.  The conversions use an LUT
together with the AVX2 gather instructions to process 8 values a
 once.  Depending on the formats and cache utilization, the new
conversions are between 1.25x to 2.2x faster than the existing
conversions.