Merge pull request #11981 from brotheramai/multimesh

Adding a sample mesh so demo code works 'out of the box'.

Author: brotheramai

tutorials/performance/using_multimesh.rst

@@ -62,6 +62,8 @@ efficient for millions of objects, but for a few thousands, GDScript should be f
         multimesh = MultiMesh.new()
         # Set the format first.
         multimesh.transform_format = MultiMesh.TRANSFORM_3D
+        # Set the mesh that will be duplicated.
+        multimesh.mesh = BoxMesh.new()
         # Then resize (otherwise, changing the format is not allowed).
         multimesh.instance_count = 10000
         # Maybe not all of them should be visible at first.