[scribus] Workaround for "partial image export"

[Peter Nermander]

> Would like opinions or possible constructive criticism on the validity of this workaround.

My first thought is: What happens to colors?

I don't think export to image uses color management, so all such
things are lost.

Also if images have some kind of transparency together with text this
gets lost. The "big image" can only have one level of transparency.

/Peter