The handling of influence vertexgroups was incomplete/inconsistent.
Some prior work was already done in 08539618cd, c452d5d9e8,
782a4c9d85, 0fd3f3c216, 434f94b254, 82ea972834, f6b820ccb7,
00968fe6db .
Now that the inverting is handled consistently, we can move that part
out of specific modifier code an into the generic retrieval function
`greasepencil::get_influence_vertex_weights` -- resulting in all
modifiers using it behaving the same.
There were some modifiers already handling
`GREASE_PENCIL_INFLUENCE_INVERT_VERTEX_GROUP`, however even those still
had one issue: if no vertexgroup was set, invert would still have been
calculated (this issue is also resolved with this PR).
Remaining modifiers that were offering influence vertexgroups (e.g.
Noise) but ignoring it now work correctly.
NOTE: there are still modifiers handling influence vertexgroups outside
of `greasepencil::get_influence_vertex_weights`:
- Armature (has own code for this in
`BKE_armature_deform_coords_with_curves`)
- Shrinkwrap (has own code for this in `shrinkwrapParams_deform`)
- Thickness (this rather special behavior is untouched)
Pull Request: https://projects.blender.org/blender/blender/pulls/133426
e4c125400f