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

source/blender/makesrna/intern/rna_curve.c

@@ -1576,6 +1576,25 @@ static void rna_def_curve(BlenderRNA *brna)
       {0, NULL, 0, NULL, NULL},
   };
 
+  static const EnumPropertyItem curve_taper_radius_mode_items[] = {
+      {CU_TAPER_RADIUS_OVERRIDE,
+       "OVERRIDE",
+       0,
+       "Override",
+       "Override the radius of the spline point with the taper radius"},
+      {CU_TAPER_RADIUS_MULTIPLY,
+       "MULTIPLY",
+       0,
+       "Multiply",
+       "Multiply the radius of the spline point by the taper radius"},
+      {CU_TAPER_RADIUS_ADD,
+       "ADD",
+       0,
+       "Add",
+       "Add the radius of the bevel point to the taper radius"},
+      {0, NULL, 0, NULL, NULL},
+  };
+
   srna = RNA_def_struct(brna, "Curve", "ID");
   RNA_def_struct_ui_text(srna, "Curve", "Curve data-block storing curves, splines and NURBS");
   RNA_def_struct_ui_icon(srna, ICON_CURVE_DATA);
@@ -1757,6 +1776,15 @@ static void rna_def_curve(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Twist Method", "The type of tilt calculation for 3D Curves");
   RNA_def_property_update(prop, 0, "rna_Curve_update_data");
 
+  prop = RNA_def_property(srna, "taper_radius_mode", PROP_ENUM, PROP_NONE);
+  RNA_def_property_enum_sdna(prop, NULL, "taper_radius_mode");
+  RNA_def_property_enum_items(prop, curve_taper_radius_mode_items);
+  RNA_def_property_ui_text(prop,
+                           "Taper Radius",
+                           "Determine how the effective radius of the spline point is computed "
+                           "when a taper object is specified");
+  RNA_def_property_update(prop, 0, "rna_Curve_update_data");
+
   prop = RNA_def_property(srna, "bevel_factor_mapping_start", PROP_ENUM, PROP_NONE);
   RNA_def_property_enum_sdna(prop, NULL, "bevfac1_mapping");
   RNA_def_property_enum_items(prop, bevfac_mapping_items);