BUG : fix eps corruption when using clipping

_get_clip_path was returning '<built-in function id>' instead
of the ps-function name like it should have been.  This was introduced
in 9b9c0c6fbdd53ec42e2a1edecc9c6895b6f7a3ef in PR #2927.

Closes #3523
Merge pull request #3526 from tacaswell/fix_eps_clip_paths

BUG : fix eps corruption when using clipping