Spline IK: support changing individual bone length via Y scaling.

Previously Spline IK provided only two choices: either scale the
length of the bone chain to fit the length of the curve, or don't
scale the bone in the Y dimension at all (ignoring effects of
actually fitting to the curve due to curvature and curve object
scale).

This patch adds a new option to use the pre-IK Y scale of the
bones to adjust their length when fitted to the curve, allowing
individual posing control over the length of the segments.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4687

release/scripts/startup/bl_ui/properties_constraint.py

@@ -794,9 +794,9 @@ class ConstraintButtonsPanel:
 
         col = layout.column()
         col.label(text="Chain Scaling:")
-        col.prop(con, "use_y_stretch")
         col.prop(con, "use_curve_radius")
 
+        layout.prop(con, "y_scale_mode")
         layout.prop(con, "xz_scale_mode")
 
         if con.xz_scale_mode == 'VOLUME_PRESERVE':