Draw a circle using world coordinates and radius.

def draw_circle(self, color, world_loc, world_radius, thickness=0):
    """Draw a circle using world coordinates and radius."""
    if world_radius > 0:
      center = self.world_to_surf.fwd_pt(world_loc).round()
      radius = max(1, int(self.world_to_surf.fwd_dist(world_radius)))
      pygame.draw.circle(self.surf, color, center, radius,
                         thickness if thickness < radius else 0)