Fix #860.
Fix #860.

This waits until the first frame is drawn before showing the canvas,
guaranteeing that if it throws an exception, the canvas is never
shown.

pranjaltale16/codeworld,alphalambda/codeworld,alphalambda/codeworld,google/codeworld,pranjaltale16/codeworld,google/codeworld,alphalambda/codeworld,google/codeworld,pranjaltale16/codeworld