GPencil Tools: Fix box-deform with multi-user data

Box deforming a grease pencil that has multiple instances now works without throwing an error.

greasepencil_tools/__init__.py

@@ -21,7 +21,7 @@ bl_info = {
 "name": "Grease Pencil Tools",
 "description": "Extra tools for Grease Pencil",
 "author": "Samuel Bernou, Antonio Vazquez, Daniel Martinez Lara, Matias Mendiola",
-"version": (1, 4, 2),
+"version": (1, 4, 3),
 "blender": (2, 91, 0),
 "location": "Sidebar > Grease Pencil > Grease Pencil Tools",
 "warning": "",

greasepencil_tools/box_deform.py

@@ -262,8 +262,22 @@ def delete_cage(cage):
 
 def apply_cage(gp_obj, cage):
     mod = gp_obj.grease_pencil_modifiers.get('tmp_lattice')
+    multi_user = None
     if mod:
+        if gp_obj.data.users > 1:
+            old = gp_obj.data
+            multi_user = old.name
+            other_user = [o for o in bpy.data.objects if o is not gp_obj and o.data is old]
+            gp_obj.data = gp_obj.data.copy()
+
         bpy.ops.object.gpencil_modifier_apply(apply_as='DATA', modifier=mod.name)
+
+        if multi_user:
+            for o in other_user: # relink
+                o.data = gp_obj.data
+            bpy.data.grease_pencils.remove(old)
+            gp_obj.data.name = multi_user
+
     else:
         print('tmp_lattice modifier not found to apply...')