Consider using Zstandard and/or LZ4 instead of Deflate

Summary

One of the issues we have with .PNG is slow read/write times. There are now new lossless open source codecs without patent concerns, such as Zstandard (maintained by Facebook) or LZ4: https://facebook.github.io/zstd/ https://github.com/lz4/lz4 Zstandard is used by the new Khronos KTX2 GPU texture format specification. I propose that it be added as an option to a future version of .PNG. The possible speedups are quite significant, and for users that read and write a lot of .PNG's as part of their data processing pipelines the speedups will be high value improvements. There are also other far simpler but even faster codecs being developed, such as .QOI's, but using this would likely require changing or not filtering the image before compression: https://news.ycombinator.com/item?id=29328750