Expose pattern and gradient construction

[eevee]

In convert, these are special syntax for special pseudo-formats. I don't know where they'd live here. Just functions in the image module?

Note that I need a way to expose a list of available patterns, as well. (#5 strikes again.)

Also, I don't think the builtin gradient construction functions allow for very fine-grained stops; can we improve on this and emulate if necessary?