Comment 10 for bug 517082

Now I have to find a way to use sp_item_scale_rel inside ImageMagick::Effect. I think it is safer than playing with the node attributes directly, since height and width are chars and we never know what we could find in it (value, value + unit?).
I've started to copy a part of transformation.cpp which is not far from what I'm looking for. But I need to investigate more and understand how sp_items and all the related stuff work first...