anchor: BezierCurve:insertControlPoint
  Insert control point as the new i-th control point. Existing control points from i onwards are pushed back by 1. Indices start with 1. Negative indices wrap around: -1 is the last control point, -2 the one before the last, etc.
  BezierCurve:insertControlPoint( x, y, i )
  x number Position of the control point along the x axis.
  y number Position of the control point along the y axis.
  i (-1) number Index of the control point.