perf(ssim): improve rgb and weber speed

[obartra]

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

[obartra]

:tada: This PR is included in version 3.3.2 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: