Further cleanup of skeleton paths.

Further cleanup of skeleton paths.

The cleanup itself is good, but emit_paths() will need enhancement.
At the moment, it assumes that all paths are closed, so it doesn't
matter where on a path it starts emitting it.  With an open path,
though, it's important to emit it starting at one end, so there will
have to be a mechanism for distinguishing open from closed paths and
behaving accordingly.