Modifiers: Vertex Weight Edit add invert curve falloff option

This commit adds the option to invert the resulting weights of the
falloff curve.

There is a workflow used by some to convert a texture mask into
vertex weights by using a custom curve and inverting the points.
This allows the same effect with a single click, and gives the modifier
more procedural functionality.

With minor UI tweaks by @mont29.

Differential Revision: https://developer.blender.org/D6899

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1350,7 +1350,9 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
 
         layout.separator()
 
-        layout.prop(md, "falloff_type")
+        row = layout.row(align=True)
+        row.prop(md, "falloff_type")
+        row.prop(md, "invert_falloff", text="", icon='ARROW_LEFTRIGHT')
         if md.falloff_type == 'CURVE':
             layout.template_curve_mapping(md, "map_curve")