Fix some odd edge cases with arc drawing.
Fix some odd edge cases with arc drawing.

Now arc(a, b, r) is the same as arc(b, a, r), and
circles with a negative radius are drawn.

venkat24/codeworld,nomeata/codeworld,alphalambda/codeworld,google/codeworld,three/codeworld,three/codeworld,venkat24/codeworld,alphalambda/codeworld,pranjaltale16/codeworld,tgdavies/codeworld,parvmor/codeworld,pranjaltale16/codeworld,tgdavies/codeworld,stefan-j/codeworld,stefan-j/codeworld,alphalambda/codeworld,stefan-j/codeworld,google/codeworld,parvmor/codeworld,google/codeworld,parvmor/codeworld,tgdavies/codeworld,three/codeworld,pranjaltale16/codeworld,venkat24/codeworld,nomeata/codeworld,nomeata/codeworld