Parallel-line indicators in graphie-helpers

This fixes a bug from the Trello boards: https://trello.com/c/zrCN2NlG/590-indicate-that-trapezoids-are-really-trapezoids-in-quadrilateral-angles

PS - I only started looking at khan-exercises this morning, and I'm not sure how to indicate that this bug's state has changed on Trello..