Port recent changes from pixelmatch JS

[mourner]

Particularly relevant changes:

• mapbox/pixelmatch@7cd740d
• Fix antialias detection on image edges pixelmatch#51
• Optimize for identical images case pixelmatch#60

cc @brunoabinader (not sure if those could affect diff, but still worth upgrading anyway)

[sergeevabc]

Err… Hello?

[mourner]

@sergeevabc if you're hinting that you're about to submit some pull requests, we'll be happy to review them 😏

[jwmcglynn]

I created a parallel port of the JavaScript library. It includes the latest changes in the JavaScript version: https://github.com/jwmcglynn/pixelmatch-cpp17

[piyush-jaiswal]

Is this issue still relevant/open?

[mourner]

Will be covered by #8 (10 years worth of performance improvements). Bringing in additional options introduced will need a separate pass.