Provide multi-theaded PNG encoding option via mtpng crate

[Shnatsel]

https://github.com/brion/mtpng provides a multi-threaded PNG encoder that is competitive with libpng even in single-threaded configurations. It would be nice to be able to use it from image.

Since multi-threading "in the background" without user control can be undesirable and encoding ratio may suffer for very small images, it's probably best to provide it as a separate encoder type so that the API client would have full control over what's happening.