Provide a new ImageSharp NuGet package which contains the image comparison utility classes

[matt-psaltis]

To support visual regression testing, I am requesting a new feature.

ImageSharp already contains Image comparison in the Test namespace located here: https://github.com/SixLabors/ImageSharp/tree/master/tests/ImageSharp.Tests/TestUtilities/ImageComparison

These methods call internal methods in the main ImageSharp library and so I'm currently resorting to reflection to do this.

Could these be moved to a separate NuGet library for public consumption?

[antonfirsov]

@SaltyDH cool that you are interested in this! :)

Unfortunately, these classes are kinda messy, and need a lot of refactor before making them public. We can only take care of this past 1.0.