v3.3.2

perf(ssim): improve rgb and weber speed (#284)

Weber algorithm improvements:
- No interim rounding
- No division
- Removed branching
- Keeps all numbers as floating points at the end

RGB to Grayscale improvements:
- Makes integer rgb calculation the default.
- Old behavior can be restored by setting the option `rgb2grayVersion`
  to 'original'
- Value changes are ~±0.001

Closes #276, closes #277, closes #278, closes #279, closes #281

BREAKING CHANGE