Replace a `StringRef` with `std::string`, so that a copy is made of the
grid item identifier.
This identifier is used on redraw, to correlate the newly-drawn items
with items from the previous redraw. Using a `StringRef` here was
problematic when there's local assets, as those can be freed & re-built
(which in the case of the report happens on save/undo). Because of this,
the reference got corrupted, and the map lookup would fail. The "old
items" map now has a copy of the identifier, ensuring it is independent
of the data it represents.
Co-authored by Julian Eisel.
Pull Request: https://projects.blender.org/blender/blender/pulls/141888
c64e13ecae