Support arc_to on Canvas

[corranwebster]

The HTML Canvas has two path operations which are currently not supported by the Canvas widget: arcTo() and roundRect(). This PR implements arc_to() in a manner compatible with the HTML Canvas. This was originally going to include support for round_rect() as well, because I thought it would need arc_to() but it turned out not to, so I've split that into a separate PR for ease of review.

Issues:

• arc_to() needs to know the current point in the path, which Android Graphics doesn't expose cleanly. We can track it manually, but that will need to Fix mid-path transforms #4106 to be merged before it could be done properly.
• arc_to() on winforms is drawing an extra line (or an extra-long line) in the test, and I can't work out what's going on. My suspicion is that it is to do with the non-standard way winforms handles paths, but I don't know where it's going wrong. Concretely, it seems to be drawing the connecting line to the first curve from (115, 10) to (80, 10) instead of (90, 10). Otherwise all is correct. This could be an existing issue with the way it draws Bézier curves or lines.

Ref #3994

To Do:

• Implement arc_to
  • ...on Android
  • ...on Winforms

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[corranwebster]

Deferring this until #4163 is completed, as I think that will make the implementation on Windows and Android much easier.