Inconsistent arc union behavior

[alex-r-bigelow]

Thanks for this awesome library!

Just reporting a few cases where very similar paths sometimes succeed, and sometimes fail, in case they're useful.

Note that, in order to actually see these paths on the demo page, tweaking the viewBox to be a bit bigger helps (these screenshots use 200x200).

I'll look closer at the code and attempt a PR in a bit. My first guess would be that there's a problem related to the SVG arc syntax itself, in that it's impossible to specify a 360-degree circle without breaking it into semicircles; see also: https://stackoverflow.com/a/10477334

---

Working Correctly

pathBoolean(
  'M68,39a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  'M112,70a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  PathBooleanOperation.Union)

---

The first path disappears, when the second is shifted slightly

pathBoolean(
  'M68,39a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  'M102,70a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  PathBooleanOperation.Union)

---

Union not performed, with another position

pathBoolean(
  'M68,39a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  'M105,73a32,32,0,1,0,-64,0a32,32,0,1,0,64,0',
  FillRule.NonZero,
  PathBooleanOperation.Union)

[r-flash]

Thank you so much for contributing! Let me know if you have any problems understanding the code. Thorough commenting and explanation of the code is still a TODO unfortunately. I'll look into your reported bugs too.