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
26ef4fa85e