The list of selected layers may be empty, which doesn't matter much
because we don't actually do much with this list in current export code.
In the code modified in this commit, we were only using existing layers
to set the type of a new layer (which seems very useless right now as
anyway the layer type has to be the image base type with or without
alpha, so a with_alpha boolean parameter would be just as good, unless
we plan to support different color model layers in a same image).