Add support for polyfaceMesh outline rendering

[AlexanderBelokon]

Old dxf documents created by AutoCAD sometimes encoded outlines as polyface meshes.

Turns out, rendering these outlines isn't that hard, so to extract some data that I needed I added their support to this package.

Would be happy if this feature can benefit other users of dxf as well!

Added tests for new code, including a sample outline encoded as a polyface mesh.

This is what this outline renders as with the patch:

[skymakerolof]

Nice work, thank you very much!