Ability to callback additional diff-engine with decoded images data

[Quramy]

Motivation

I'm working on https://reg-viz.github.io/x-img-diff-js and want to integrate it to reg's toolchain.

x-img-diff-js requires decoded input images such as:

const image = {
  width: 100,
  height: 100,
  data: new Uint8Array(buffer),  // 32-bit 10K sized array
}

For now, img-diff-js has responsibility to decode PNG/JPEG/etc images to canonical(the above) format, so it's nice to pass decoded data to the additional diff-engines.