An option to "don't erase first" when putting a prop on top of another
Right now, if you put a prop on top of another one, the finger erases in the area where the new prop is going that overlaps the old one.
BUT this looks silly if there are no lines there in the first place - for example putting a picture inside a "square".
We should be able to say "don't erase first".
Of course, we need this on a per asset basis, because when there _are_ lines underneath, it is good to erase them!