Keep previous output during refresh

As we tweak the settings in #photo-ai it would help to not reset to the original image on the preview side. Persistence of vision makes it much easier to notice the changes, which is reset when the image preview is reset.

The simplest and most needed solution would be to persist the previous edits while the updates are rendering, then display the updates without flashing back to the original.

Even better would be to be able to switch back and forth between the previous values and the new values. A light undo/redo stack, or perhaps even a full history some day.