Draws an arc. The current pen is used for the outline and the current
brush for filling the shape.
(send a-wx:canvas draw-arc x1y1x2y2xcyc)->void x1 : inexact real y1 : inexact real x2 : inexact real y2 : inexact real xc : inexact real yc : inexact real
Draws an arc, centred on (xc, yc) with starting point
(x1, y1) and ending at (x2, y2).