Applying a modifier on Grease Pencil objects tries to maintain the
original layer order by inserting each layer after the previous. This
ignores layer groups and all layers get move to the root group.
This patch replaces the simple `previous_node` with a per-group pointer
map, which maintains order within each group only.
The code is also creating new layers when the original node of the same
name is a group. This was creating duplicate names which are not allowed
and break various things (layer editing buttons, panel open/close
toggles, etc.). Now these new layer names are made unique to avoid a
name conflict.
One remaining issue is that mixed layer/group ordering is lost: All the
layers get pushed to the bottom of the stack, due to the way
`geometry::merge_layers` ignores layer groups. The existing groups in
the orig data are not sorted along with the evaluated layers, so they
move to the top of the stack. This requires changes to `merge_layers`
and should be handled separately.
Pull Request: https://projects.blender.org/blender/blender/pulls/129943
9a03f283e8