Geometry Nodes: Avoid potential copy of attribute in simulation bake mixing

Only retrieve a mutable copy of the attribute if we're actually able to change it.
If topology changes and there are no IDs, we can't mix the attribute and we
should avoid retrieving it in case it's shared. This is more of a hypothetical
change, I didn't actually observe a real world performance change.

Pull Request: https://projects.blender.org/blender/blender/pulls/129811

source/blender/geometry/intern/mix_geometries.cc

@@ -97,18 +97,20 @@ static void mix_attributes(bke::MutableAttributeAccessor attributes_a,
     if (sharing_info_equal(attribute_a.sharing_info, attribute_b.sharing_info)) {
       continue;
     }
-    bke::GSpanAttributeWriter dst = attributes_a.lookup_for_write_span(id);
     if (!index_map.is_empty()) {
+      bke::GSpanAttributeWriter dst = attributes_a.lookup_for_write_span(id);
       /* If there's an ID attribute, use its values to mix with potentially changed indices. */
       mix_with_indices(dst.span, *attribute_b, index_map, factor);
+      dst.finish();
     }
     else if (attributes_a.domain_size(domain) == b_attributes.domain_size(domain)) {
+      bke::GSpanAttributeWriter dst = attributes_a.lookup_for_write_span(id);
       /* With no ID attribute to find matching elements, we can only support mixing when the domain
        * size (topology) is the same. Other options like mixing just the start of arrays might work
        * too, but give bad results too. */
       mix(dst.span, attribute_b.varray, factor);
+      dst.finish();
     }
-    dst.finish();
   }
 }