Draws a circle that fits inside a square with its top-left corner at x,y and a width/height of the circle's diameter.
draw_circle(x, y, diameter)
draw_circle(x, y, diameter, fill_in=True)
x: x coordinate of the top-left corner of the enclosing square
y: y coordinate of the top-left corner of the enclosing square
diameter: diameter of the circle (also the width and height of enclosing square)
fill_in: Boolean that selects if the triangle will be filled in (white). Default to True.