Blender Git Loki

Bugfix #26424

More problems with Undo and Render Slots (Image editor)

- Undo storage for operator is now back, but only when new
buffers were added (not when viewing existing)

- A real bug: On undo/redo, the stored buffers were never
retrieved, but always freed entirely.

Note however that when you undo back to a state before you
rendered (or added slots), the render buffers that didn't
exist back then also get freed. A redo doesn't bring it back.