More accurate Z sorting of Shapes

[bendablegears]

Zdog now correctly discards the final overlapping point in a closed path. Fixes #26 for Ellipses, Rounded Rectangles, and custom path Shapes. Fixing that caused Z fighting with co-planar Hemispheres and Cones, so those now calculate a better average Z.

[desandro]

A lot of great work here. Thank you so much. I've got some ideas on how to refactor this. Are you interested in implementing them? Otherwise, I can pull your changes and handle it myself.

• Run npm install and make lint to lint JS
• Add Vector.isSame method and use it
• Several shapes don't actually need calculation, as the renderOrigin.z could be used as the center of the shape: Rect, RoundedRect, Polygon, Box, Cylinder. I wonder if its worth hard-coding this in.

[bendablegears]

I addressed the first two points in a second commit. I'll leave the decision on the other shapes' Z sorting to you.

[desandro]

Thanks so much for this work here. This will go out in the next release of Zdog. If you like stickers, please email me yo@metafizzy.co so I can send some your way.