Transfer Terminal

[JosephCottam]

Images are a terminal type, but so are contours and maybe other fancy arrangements.

It would be nice if renderer.transfer wasn't tied down to aggregates->aggregates transitions. This changes the AR equations

• This might be done as a tagging interface (specialized.transfer.terminal) but that might some transfer types difficult because they may be terminal under some type-parameters but not others.
• This might be done as a parameter type on transfer, but right now transfer only really knows about the point-transition so it would require a major re-work.

[JosephCottam]

Not going to look into this for a long time. It really changes AR in ways that move it out of its current sweet spot. Terminal values are currently handled by special aggregate types (images are color aggregates, contours are stored as a list in the ContourAggregates) and may be extracted by functions that need them. A more general solution requires great violence to the framework, and I'm not sure its particularly valuable.