Indexed vertices API

[mourner]

Changes the API to return indexed vertices. Closes #4.

We need to evaluate the approach in the real world first before making a change like this because it comes with a performance cost of ~15-20%.

This also breaks the 3D vertices use cases, we need to think about it API-wise.

[mourner]

I should probably also make this optional.

[kkaefer]

what's the status of this?

[mourner]

@kkaefer never tried it in practice but it works on the earcut end and I can quickly rebase the branch to work on master. Do you think it should be the default behavior?

[kkaefer]

@mourner we're exclusively doing indexed draw calls in native, so we'd need some way of doing this anyway.

[mourner]

@kkaefer OK, I'll look if this can be merged to master in a way that doesn't degrade performance (e.g. making it optional).

[mourner]

Another pull request for the same: #11