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Lattice Modifier add invert vgroup option

Adds the invert vertex group option to the Lattice modifier.

Adds a short flag and modifies the existing char padding for the correct amount.
Adds a .invert_vgroup to the LatticeDeformUserdata.
Passes the flag into the lattice_deform_verts function where the weights around found and used.
For the other calls of lattice_deform_verts function they pass in NULL for the flag in the same way they pass NULL for the vgroup name.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D6747