fix(ui): after canceling a filter, layer loses its content

[psychedelicious]

Summary

Fixes a bug I noticed when testing graphs. This was the sequence of events that triggered the issue:

1. Start new canvas
2. Add control layer
3. Filter it
4. Apply filter
5. Change control layer model
6. Filter automatically starts
7. Cancel it
8. Generate
9. The control layer is used for generation, but it is all black, regardless of its content after the first filter.

The issue is related to how the filter module passes the filtered image back to the layer when it's done.

The filtered image is reparented via a konva API, but we did not reset the filter module's reference to the image. Later on, the filter module sees this image in its state and erroneously assumes it owns the image. It does some cleanup and destroys the image, inadvertently destroying it in layer.

The solution is to ensure that when filtering completes, nulls out that reference to the reparented image.

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Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)