Fix broken trace behavior for non-closed paths

[jeffc]

Fixes two bugs in trace operations for open paths:

1. Allows length-2 open paths (ie, a single segment), which were previously filtered out
2. Checks the direction ("winding order" equivalent) of open paths and corrects them if necessary. Open paths are generated by linking segments selected by the user, which could lead to arbitrary flipping of segment direction. By ensuring that the "right-handed" normal points out of the part, we ensure that "outside" offsets do indeed go out (and "inside" offsets go in).

I've attached an example workspace that traces a single segment on a part. In the current version of kiri:moto, no toolpaths are generated. If you apply the first commit in this PR, the toolpath gets generated inside the part even though "outside" is selected. When you apply both commits, it works as expected.

note: I discovered #510 while writing this, which is why .right got switched to .left in polygons.js. I figured I'd stick with the current convention rather than try to fix the whole thing as part of this PR.

workspace_key_offset_example.zip