[PNG] zstd supports restart markers

Hello everyone,

We're looking into multithreaded PNG encoding/decoding, which would rely on
deflate's restart markers.
We're also looking into other compressors, like zstd.

Zstandard supports frames, which have the same effect as deflate's restart
markers.
https://github.com/facebook/zstd/blob/3bee41a70eaf343fbcae3637b3f6edbe52f35ed8/doc/zstd_compression_format.md#frames

Just spreading the information.