Plz add Google's Guetzli encoder in addition to libjpeg

[Marsu42]

I've become a supporter and user of (esp. pre-processed lossless) webp, but with more computing power there are obviously still improvements even in ancient formats... behold, Google strikes again: https://research.googleblog.com/2017/03 ... -jpeg.html

Guetzli is a JPEG encoder that aims for excellent compression density at high visual quality. Guetzli-generated images are typically 20-30% smaller than images of equivalent quality generated by libjpeg.

Guetzli: Perceptually Guided JPEG Encoder: https://arxiv.org/abs/1703.04421
75% of users prefer Guetzli JPEG over same-sized libjpeg: https://arxiv.org/abs/1703.04416

Get the code here: https://github.com/google/guetzli/
Download binaries hiere: https://github.com/google/guetzli/releases

At some point in time, it would be nice to have Guetzli inside IM like libjpeg ... but as development is progressing quickly and atm apart from the quality there are zero config options a simple link to the cli exe (like with jxrenc an so on) would be nice.

Drawbacks: Guetzli generates only sequential (nonprogressive) JPEGs, and only for high quality >= 84 libjpeg equivalent setting.