Exporting one png per frame causes an exception to be thrown.

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To Repro:

1. Create a multiframe document.
2. Choose File->Export.
3. In the formats menu of the file dialogue, choose One PNG image for animation frame (*-XXX.png).
4. Click Save and observe the results.

The problem stems from lib/animation.py, line 192 - this needs to be cel.save_as_png(filename, *doc_bbox, **kwargs), but there's something else required here - by default, a layer only seems to know to export itself, but not the backdrop image, so some additional tinkering in lib/layer.py might be needed.

I can try to look into it this week and submit a patch.

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Note that this won't fix the background problem, which really should be broken out into a separate ticket (due to complexity and being in a different part of the code).