Fix #121666: Modifying UVs of duplicate affects the original object

Caused by 7eee378eccc8.

Since the implicit sharing commit, uv data was not unshared when
accessed. Fix along the ines of 34b44878449d (same downside of not being
able to tell if the data is just read -- or written to).

Pull Request: https://projects.blender.org/blender/blender/pulls/121786

source/blender/makesrna/intern/rna_mesh.cc

@@ -941,10 +941,12 @@ static void rna_MeshUVLoopLayer_data_begin(CollectionPropertyIterator *iter, Poi
 {
   Mesh *mesh = rna_mesh(ptr);
   CustomDataLayer *layer = (CustomDataLayer *)ptr->data;
+  const int length = (mesh->runtime->edit_mesh) ? 0 : mesh->corners_num;
+  CustomData_ensure_data_is_mutable(layer, length);
   rna_iterator_array_begin(iter,
                            layer->data,
                            sizeof(float[2]),
-                           (mesh->runtime->edit_mesh) ? 0 : mesh->corners_num,
+                           length,
                            0,
                            nullptr);
 }