Draws an arc to the screen.
:width - The width of the arc (number)
:height - The height of the arc (number)
:start - Angle to start the arc, in radians (number)
:stop - Angle to stop the arc, in radians (number)
[:arc {:x 200, :y 0, :width 200, :height 200, :start 0, :stop 3.14}]