3D polyline support?

[Jakobovski]

Are 3D polylines supported?

[andrewharvey]

Not implemented at the moment. We already have two different encodings supported (regular polyline and polyline6 which has an extra decimal of precision), so I don't see any issue with supporting a polyline3d and polyline63d.

[noc7c9]

Hi, would you accept a PR for this functionality?