Adds the Invert Vertex Group weight option to the Cast modifier.
Uses the same setup as similar modifiers invert weight.
Adds a boolean invert property next to the vertex group string in the modifier
and subtracts the current vertex weight from 1.0f if it is turned on.
Differential Revision: https://developer.blender.org/D6671
Minor modifications by @mont29.
1094e56041