Curves: add taper mode option

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.

Ref D10779

release/scripts/startup/bl_ui/properties_data_curve.py

@@ -175,6 +175,7 @@ class DATA_PT_geometry_curve(CurveButtonsPanelCurve, Panel):
         sub.prop(curve, "extrude")
 
         col.prop(curve, "taper_object")
+        col.prop(curve, "taper_radius_mode")
 
         if type(curve) is not TextCurve:
             # This setting makes no sense for texts, since we have no control over start/end of the bevel object curve.