Curve-based control for child path tapering.

This is an alternative method to the current fixed function with a
clump factor and "shape" parameter. This function is quite limited and
does not give the desired result in many cases (e.g. long, parallel
rasta strands are problematic). So rather than trying to add more
parameters there is now a fully user-defined optional curve for setting
the tapering shape.

release/scripts/startup/bl_ui/properties_particle.py

@@ -1167,8 +1167,12 @@ class PARTICLE_PT_children(ParticleButtonsPanel, Panel):
         col.label(text="Effects:")
 
         sub = col.column(align=True)
-        sub.prop(part, "clump_factor", slider=True)
-        sub.prop(part, "clump_shape", slider=True)
+        sub.prop(part, "use_clump_curve")
+        if part.use_clump_curve:
+            sub.template_curve_mapping(part, "clump_curve")
+        else:
+            sub.prop(part, "clump_factor", slider=True)
+            sub.prop(part, "clump_shape", slider=True)
 
         sub = col.column(align=True)
         sub.prop(part, "child_length", slider=True)