[enhancement]: During generation, keep the last-displayed image in Progress Image cached/visible

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

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### What should this feature add?

It would be nice if the Progress Image node could, during generation, continue showing a cached copy of whatever the last image was that it displayed, so that image remains visible even if it is not saved to the gallery. This way we could always be looking at an image from the current generation while subsequent processing is occurring, rather than the current behavior where the preview image keeps reverting to the last saved image in the gallery in between denoising steps.

To illustrate the issue, consider an upscaling process in which three images are created sequentially from one another, at sizes Small, Medium, and Large. If the Small and Medium images are not saved to gallery, then as soon as each denoising step that creates them finishes, the Progress Image node reverts to displaying the last image in the gallery until the next denoising step begins. So, instead of watching the image develop smoothly from small->medium->large, the viewer watches each size image develop independently and then immediately disappear, replaced by whatever image came out of the previous Invoke.

Feature request opened after discussion with @psychedelicious

### Alternatives

Currently the basic workflow can be achieved by enabling "Save to Gallery" on all the intermediate upscaling images so they remain visible as processing continues. However, this may not always be necessary or ideal.

### Aditional Content

_No response_