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Cast modifier: Add invert vgroup option.

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.