Copying filters over from one image to a partial copy was broken on
multiple level by doing it on an additional loop, and assuming that the
originally selected layers and the newly created ones had the same
structure:
1. First the actually copied layers may be more numerous. For instance,
we'd also copy parent layers. Typically copying a layer under a layer
group, both the layer and its parent layer group would be in the clip
image.
2. Second, because this structure may change, the assumptions made so
that filters were not copied to the right image.
3. And as a last consequence, sometimes we could have crashes, assuming
the same structure and therefore directly dereferencing a NULL
pointer. I had such a crash when copy-pasting a layer group, while
also selecting its children layers (though this crash also got hidden
by my previous commit, but just by chance).
Instead just copy filters in the same time as layers are copied, so that
we can easily associate the filter from the correct original layer we
copied from.
690391b985