arc draws obese degenerate arcs

The arc function draws an arc when the start and stop angles are equal. In "more normal" Processing.js environments it does not. See a demo at https://www.khanacademy.org/computer-programming/obese-arcs/6704678451150848

Start by reproducing the behavior in the linked Khan Academy demo, then locate the JavaScript implementation of the arc function in the live editor. Confirm how equal start and stop angles are handled and verify that the completed behavior no longer draws an arc for that case while normal arcs still render.

Written by the indexing model from the issue text.