Currently, when a taper object is specified, the radius of the spline is
ignored. This patch adds a new option to control how the taper object
affect the effective radius of the spline. The option allow three modes
of operation:

• Override: The old method. The radius of the spline is ignored and overridden.
• Multiply: The radius of the spline is multiplied by the taper radius.
• Add: The radius of the spline is added to the taper radius.