Fix T96233: Crash with gpencil data with vertex groups

Blender crashes when a multi-user grease pencil object has vertex
groups and is modified by modifiers, layer transform or parenting.

The fix makes sure that we copy the vertex group names list.

Reviewed By: antoniov

Maniphest Tasks: T96233

Differential Revision: https://developer.blender.org/D14275

source/blender/blenkernel/intern/gpencil_modifier.c

@@ -28,6 +28,7 @@
 #include "DNA_screen_types.h"
 
 #include "BKE_colortools.h"
+#include "BKE_deform.h"
 #include "BKE_gpencil.h"
 #include "BKE_gpencil_geom.h"
 #include "BKE_gpencil_modifier.h"
@@ -632,6 +633,8 @@ static bGPdata *gpencil_copy_structure_for_eval(bGPdata *gpd)
     gpd_eval->mat = MEM_dupallocN(gpd->mat);
   }
 
+  BKE_defgroup_copy_list(&gpd_eval->vertex_group_names, &gpd->vertex_group_names);
+
   /* Duplicate structure: layers and frames without strokes. */
   LISTBASE_FOREACH (bGPDlayer *, gpl_orig, &gpd->layers) {
     bGPDlayer *gpl_eval = BKE_gpencil_layer_duplicate(gpl_orig, true, false);