rate distortion optimized (RDO) PNG encoder

I mentioned this repo in today's meeting:
https://github.com/richgel999/rdopng

Example PNG images encoded in this way:
https://github.com/richgel999/rdopng/wiki/Examples

rdopng uses techniques from the video and GPU texture compression world to
create substantially smaller true color (24bpp) or RGBA (32bpp) PNG's. It
utilizes the Oklab colorspace to predict visual distortion, and a Deflate
model to predict bitrate.

It also supports RDO encoding to the .QOI and LZ4 image formats. The
downside is slow encoding because the search space is large.

-Rich