Envelope deformation

[mgsloan]

Continuation of this pull:
#40

Here's an interactive demo based on byorgey's test code: https://github.com/mgsloan/gtk-toy-diagrams/blob/master/examples/Extrude.hs

https://github.com/mgsloan/gtk-toy-diagrams/blob/master/screens/extrude.png

[byorgey]

Cool, I look forward to playing with it soon. So, just to be clear, this one replaces #40, i.e. #40 should be closed now, right? ...oh, hehe, looks like I was writing this comment at exactly the same time that you were closing #40. =)

[mgsloan]

Not sure why the first two commits are on there - I think it's due to a rebase. Since they're already in the repo it'll probably merge fine.

[byorgey]

Very cool, I finally got everything building to play with the interactive demo. =) Looks good to me! And the old tests from #40 (e.g. the extruded circle that looked like it was being extruded too far, etc.) now seem to work fine.