Exporting ORA images involves first creating PNGs of
the layers. To do this, we create separate images for
each layer with Gimp.Image.new ().
However, if the original image was indexed, we lose
the palette when passing the temporary image to
the PDB call. This caused us to try saving a NULL
palette, which resulted in the crash.
This patch adds a check if the image is indexed, and
copies over the palette to the temporary image.
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