If we parent with type `PAR_ARMATURE` (where vertexgroups are already
set up and named correctly according to the corresponding bones), we
still need an armature modifier. This just wasnt added.
In contrast to meshes [which add their armature modifier early in
`ED_object_parent_set`], grease pencil used to do this (adding the
armature modifier) in `ED_gpencil_add_armature_weights`.
Now split ED_gpencil_add_armature_weights in two:
- ED_gpencil_add_armature
- ED_gpencil_add_armature_weights (which calls ED_gpencil_add_armature)
- use ED_gpencil_add_armature for the PAR_ARMATURE case
Maniphest Tasks: T76416
Differential Revision: https://developer.blender.org/D7625
7043f8b815