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