Specially checks correctness of the API semantics:
* no move happens when a path is already opened with move-to.
* no path will be left open and close-path will happen at the end of opened paths.
* no path opens with a move-to and will be closed with no length.
* paths start and ending points matches.
* no line/quadratic/cubic command will be issued when no path is started.
Ebrahim Byagowi