Return actual canvas coordinates whenever an item is added.

[cryptoquick]

Isomer.add presently returns nothing, so I figured that was an excellent opportunity to return the canvas point data that's usually lost.

This is super important for making click detection work in https://github.com/cryptoquick/cubes and I'd imagine other people might find it handy too.

[cryptoquick]

I just realized, I used concat incorrectly here. Concat returns a new array... This should be re-assigned. I'll get to that later tonight. 😸