Format:
circle (int_x, int_y, int_radius, int_colour);
Draw circle with centre at coordinates (x,y) and specified radius. If any part of the circle is outside of the screen boundaries, only the visible bit is drawn.
If the colour is specified as negative number, only the outline of the circle will be drawn, otherwise if the colour is a positive number, the circle will be solid.
The function has no effect in the console or on text-only displays.