Implement to_kurbo on Contour

[simoncozens]

This adds a to_kurbo method on a Contour, as discussed in #110.

Most of this has been tested in fonttools-rs, apart from the implied on-curve stuff because I can't find any UFOs that do that.

[madig]

I also wrote something: https://github.com/linebender/norad/blob/8c9038b/examples/letterspacer.rs#L523-L680. Assembles PathEls instead of calling drawing methods.

[cmyr]

Sorry I think my coffee just kicked in, so there's way more feedback here than is realistically necessary. Feel free to ignore most of it at your discretion; I thought it might be helpful to see how I would think through some of these problems in a slightly more idiomatic way. :)

I'll give you a chance to respond, but I'm happy to merge this more-or-less as is.

[cmyr]

This doesn't need to be fixed here, but I want to discourage using ErrorKind as an actual error type; it's intended more to indicate the cause of some other error type, such as a parsing error or a validation error. I believe we do use it like this elsewhere in the code, but I'd like to rectify that at some point.

[simoncozens]

I'm going to leave this part for now, because I don't think there's a better error type right now - things which happen when using the curve are not exactly parsing, writing, or spec errors - and I think I'll leave you to design the error hierarchy you want to see.

[cmyr]

yep that's fine!

[simoncozens]

This is fantastic feedback, and will really help me think about idiomatic Rust. :-) Thank you! I shall work on addressing the issues above.

[cmyr]

Cool, thanks for revisiting this @simoncozens. I'm going to make a couple of doc tweaks and then merge, and I'll look into the error types before releasing 0.4.0.

Another high-level idiomatic-rust thought, here: I think the ideal implementation of this method would return an impl Iterator<Item=kurbo::PathEl>, instead of a kurbo::BezPath. A BezPath is just a Vec<PathEl> under the hood, and by having this return an iterator, you could do fun things like take the elements and pass them directly to some drawing code, or accumulate multiple contours into a single BezPath, all without needing any intermediate allocation.

In order to fully avoid allocation, the other thing we would need to get rid of is the VecDeque. For this I would probably add some custom helper type, like:

enum OffCurves {
   None,
   One(Point),
   Two(Point, Point),
}

impl OffCurves {
    fn points(&self) -> &[Point];
    fn push(&mut self, point: Point) -> Result<(), ErrorKind>;
    fn clear(&mut self);
}

I may go ahead and play around with this after merging, because I think it would be fun 😬