Fix #135711: Setting Grease Pencil modifier materials are not refcounted

When setting these materials, then removing the modifier, usercount
would be decreased, resulting in possible dataloss on reload

Affected were:
- all Grease Pencil modifiers material influence materials
- `LineartModifier` `target_material`
- `OutlineModifier`  `outline_material`

These were all using `IDWALK_CB_USER` which ends up decrementing
usercount in `modifier_free_data_id_us_cb` when the modifier is removed

So to resolve, decrement/increment material usercount in
`rna_GreasePencilModifier_material_set` appropriately

NOTE: previously, it was also doing `id_lib_extern` on the object?!
(should be on the material, no?)

NOTE: still not 100% sure where we actually use refcounting (esp. in
modifiers) and where we dont, another alternative is to just drop it and
use IDWALK_CB_NOP` (instead of `IDWALK_CB_USER`) for these materials.

Pull Request: https://projects.blender.org/blender/blender/pulls/135729

source/blender/makesrna/intern/rna_modifier.cc

@@ -2101,10 +2101,15 @@ static void rna_GreasePencilModifier_material_set(PointerRNA *ptr,
                                                   Material **ma_target)
 {
   Object *ob = reinterpret_cast<Object *>(ptr->owner_id);
+  Material *ma_old = *ma_target;
   Material *ma = reinterpret_cast<Material *>(value.data);
 
   if (ma == nullptr || BKE_object_material_index_get(ob, ma) != -1) {
-    id_lib_extern(reinterpret_cast<ID *>(ob));
+    id_us_min(&ma_old->id);
+    id_us_plus_no_lib(&ma->id);
+    if (!ID_IS_LINKED(&ob->id)) {
+      id_lib_extern(&ma->id);
+    }
     *ma_target = ma;
   }
   else {