Fails for newer versions of martinez-polygon-clipping

[ianstormtaylor]

This isn't technically a current problem, but it will be whenever the package updates its internal martinez-polygon-clipping dependency.

I had a setup where I was accidentally leaving the martinez-polygon-clipping dependency unpegged and it was importing the newest ~0.7 version and returning weird outputs with non-conforming polygon arrays when union was called internally. I'd end up with a return value of:

[
  [number, number][],
  [number, number],
  [number, number],
  [number, number],
  [number, number],
  ...
]

Instead of:

[number, number][][]

And it took me forever to track down that the internal dependency was unpegged.

Anyways, not a current issue, but it seems like the jump from [email protected] to [email protected] might break things.