New port: graphics/guetzli

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 generates only sequential (nonprogressive) JPEGs due to faster
decompression speeds they offer.

WWW: https://github.com/google/guetzli

PR: 217872
Approved by: lme (mentor)
Submitted by: Yuri Victorovich <yuri@rawbw.com>
Differential Revision: https://reviews.freebsd.org/D10074