Multi-threading with PNG Restart Marker

[randy408]

This extension introduces restart markers for PNG images with a new chunk and a backward-compatible encoding method, it enables parallel decoding and encoding of image data.

https://github.com/libspng/png-restart-marker

This will probably be the only type of multi-threading implemented for this library, it's designed to be fairly predictable in terms of performance and memory overhead.

It's still in its early stages and the format may change, will have to experiment before settling on a final spec and implementation.