Specifies vertex coordinates for curves. This function may only be used between
PendShape and only when POLYGON parameter specified to
PbeginShape. The first and last points in a series of
PcurveVertex lines will be used to guide the beginning and end of a the curve. A minimum of four points is required to draw a tiny curve between the second and third points. Adding a fifth point with
PcurveVertex will draw the curve between the second, third, and fourth points. The
PcurveVertex function is an implementation of Catmull-Rom splines.