core: Rework OutputTransform type

In some cases, it's desired to know what the inverse transform of a
given output transform is. It's possible to make it work by providing
helper functions, but we tend to avoid doing so.

This change converts the OutputTransform from an enum to a class so it's
possible to have both data + methods in the same type. Unfortunately,
unlike Rust, C++ provides no way to attach methods to enums, classes and
structs is the only way to go.